I'm playing a level 17 fighter in a long-term campaign. I've had a magic weapon that becomes any weapon as a bonus action since around level 10 and I've probably switched it from greatsword to glaive twice in 7 levels in order to get a little reach. It's just not worth giving up Great Weapon Master to make it a longsword, especially since it takes a whole action to don a shield, and the +2 to AC often isn't worth it at high level. It's completely worthless to make it a longbow, because I'm a Str fighter with other things to invest in like Con and Int. I can literally access the entire suite of weapons in the game as a bonus action, but it's rarely every optimal to lose my feat investment and not deal maximum damage in order to end the fight.
In 5e, there's no mechanical incentive to switch weapons mid combat because big damage is how you win every fight if you're a weapon user. Adding masteries might be a thing that can change that, but stacking masteries on the one weapon that deals the most damage is probably going to be the best solution once again. The battle master worked well by having things like pushing and tripping moves that worked with any weapon, because that's how most weapon users actually play the game.