The following is strictly my opinion, and has served me well for 5 decades of riding, 31 street motorcycles, I'm guessing about 800,000 miles on two wheels, and having spent 17 years working in the motorcycle industry.... of course YMMV.
I think there may be some confusion as to what "lugging" actually is. If you are running so low of rpms in too high of a gear that your engine can't accelerate cleanly when gas is applied, or if it bucks or makes a pinging sound, regardless if on a flat or going up a hill, that's lugging. At least that's what I was always taught.
Running at the lowest rpm in the tallest gear that your bike will pull cleanly and still accelerate, regardless if on a flat or going up a hill is not lugging.
How many rpms is required to avoid lugging? Well, that varies on conditions, but there is no specific number. if you have any experience at all, you will know it when you feel it or experience it. It would generally require more rpms to avoid lugging when going up a step hill and therefore require being in a lower gear than it would on a level surface. We all know that, you downshift to go up hill, where there is additional strain on the motor. It takes very little horsepower/torque/rpms to maintain a given speed on a level flat surface.
Let's take a close look at the dyno chart for a 2016 FJR 1300 ES
Look at the blue torque line, you will see it rises to approx 3500 rpm where it hits approx 82 lb ft of torque. Then it dips (just to make sure this was not an anomaly I checked several dyno charts, they all reflected this). From 3500 rpms it dips, only to rise again to the same 82 lb ft mark, at approx 5200 rpms. It then rises to a peak high of 89 lb ft at 6800 rpms. 3500 rpms and 5200 rpms are both the same, about 7 lb ft below peak torque.
(There is no similar dip in the horsepower curve and it rises rather linear from 3000 rpms to a peak at 8100 rpms.)
I find it amazing and telling that on the 6 speed trans, Yamaha geared the bike to run 3500 rpms at 75 mph. At the first torque peak. 3500 rpms in 6th gear is certainly not lugging the engine, and 6th gear on the freeway at 75 mph is quite appropriate. (Actually the speed limit on the expressway where I live is 65 MPH..70 mph about 30 miles south of me in Kentucky). Something one might do for hours or days running on the freeway from say "back east" to "out west". being a sport-tourer, the cruise control is obviously designed for this situation, running in 6th at 75 mph at 3500 rpms.
interesting that on my 6 speed CB 1100, 75 mph is also at 3500 rpms.
Running 55 mph on back roads can also be done at 3500 rpms but that would require being in a lower gear. 4th on my bike.
I've actually found where I live and ride, the FJR will cleanly pull 6th gear down to about 2000 rpms and accelerate cleanly without bucking or pinging although I will generally be in 4th at those rpms, but the FJR doesn't complain when I inadvertently find myself in that situation. Sometimes I find myself on a 2 lane state highway, running for miles at 55 mph with the cruise on in 6th and come to a small town that bumps the speed limit to 35 mph for a 1/2 mile or so. I kick off the cruise by lightly touching my clutch lever, back off the throttle and roll thru town, accelerate out the other side back to 55 mph, go to up shift and find out I forgot to downshift and went thru the town in 6th. No fuss from the FJR. Most of the time I find myself in about the 3200 rpm area when out riding. Depending on speed that would be in 4th,5th or 6th. The engine doesn't care. It's the same on my CB1100 (and on my younger brother's T-120 Triumph btw).
Of course more rpms, in lower gears offers brisker acceleration. I've hear people say it is best to shift at peak torque. On the FJR that's 6800 rpms. It's rare I am going to be in a situation where I live and ride that shifting at 6800 rpms is feasible. How fast would you be going in 3rd, 4th or 5th gear at 6800 rpms? I'm sure it's fun to run it up there every once in awhile on a nice flat road with open sight lines but I don't live or ride there for the most part. I know you could keep it in 2nd gear and run those rpms shooting from one curve to another before backing off and doing it again for the next curve but I don't ride like that either, or live where the corners (curves) facilitate that. 6800 in 2nd might actually be exceeding the speed limit ... by a bunch. I'll have to check that out sometime. Maybe someone knows and I won't have to lol. I've heard others say they like running it up to redline which on the FJR is 9,000 rpms, or at peak horsepower (which is actually before redline @ 8100 rpms). Again, not very feasible where I live and ride, and just not my style anymore either. It's funny, we were always taught that treating a motor like that was called "flogging".
Funny how opinions change over the years.
Regardless, whether you ride at low rpms ( as long as the motor isn't bucking, stumbling or pinging) or higher rpms (as high as redline), modern motors are made for this. They just don't care. Modern electronics, and fueling systems take all this into account, adjusting timing and fueling instantaneously to compensate for rpms, load, gearing, engine temp, ambient air temp,altitude, spark duration etc etc. A miracle really. You have to be doing something really stupid to hurt a modern motorcycle engine.
Change engine oil and filter at appropriate intervals with whichever combo you feel comfortable with, run the correct grade of fuel as required by the manufacturer, keep on top of other maintenance items as required and for the most part you and your motorcycle will or can have a happy life together for at least 100,000 miles, probably a lot more if so desired, regardless of whether you enjoy riding around at 3500 rpms or 8000 rpms.
Like I said, strictly my opinion and YMMV. Anecdotal evidence is strictly that ..anecdotal, and one persons anecdotal evidence/experience may be the exact opposite of another's. This is mine.