Better pickup is what makes the newer locos better not the flywheels.
Harold
Well Harold, if we're talking about your 4-4-0, I think I've already agreed with you. As for everyone else's models, I don't believe so. Take my PA's for example. They had all wheel pickup, and perform better with the flywheels than without. I have lots of brass steamers that benefit from flywheels installed, and some that don't due to insufficient room for an effective one. Tender pickups do definitely help, and I have them on my short wheelbase locos, like the 2-6-0's and the 4-4-2's.
This is all a matter of preference, of course, and to each his own, but I would be reluctant to use the words "never" and "always" in a sentence about anything in this hobby.
If however,you can prove across a broad spectrum of models that flywheels don't help, I'd suggest you contact Athearn, Atlas, Walthers, Bachman, BLI, MTH, Rivarossi, Stewart Hobbies, Overland, Sunset, Glacier Park, and the rest, and let them know that they're wasting a ton of money putting flywheels in almost everything they make. Or maybe they know something you don't