One not so minor difference between say steam and diesel locomotives and brifter vs friction DT shifters. The locomotive swap is seamless. Diesels do everything the steam ones did, just more safely and easily. (And burned a now more convenient fuel source.) Trains don't especially care what the locomotive is that pulls them.
By contrast, cassettes and FWs care very much what brifter is used to shift them. Actually, the real comparison would be track gauges. Get that wrong and the train is going nowhere. But friction shifters don't care! They don't care what brand you are running, what model, number of cogs, cog spacing ... Just - is there enough pull? YOU CAN DIG DEEP AND PULL OUT THAT 40 YEAR OLD WHEEL