So, if it had been "you can switch when you gain a level" instead of "at the end of a long rest" then it'd have been an identical feature for you? Did you switch because of gaining a Fighting Style? Or, rather, you choose Heavy Crossbow at level 1 only because it was mechanically better than Dagger even though you already planned to switch once you got Throwing or TWF? And you're only plans to switch going forward are if you find a magical weapon that's better than what you've got now?
For clarity, I switched from Crossbow to Hand Axe and yes Crossbow was better at level 1, Hand Axe was better at level 2.
You don't think that's the same as saying you're essentially never changing your mastery? You're not planning on changing on long rests really at all.
If it had been "you can switch when you gain a level" yes it would have been the same .... this time, but I am also probably going to switch the first time I get a magic melee weapon, whether or not that is after a level.
But, do you think martial characters should have to choose between mastery and magic items then? Do you think that's the design intention of, say, fighting styles and weapon-specific feats?
Yes I think that is exactly the design, especially for the Fighter class, and Crawford even commented they would be using lots of different weapons.
Furthermore, do you think it's good DMing if you take short sword mastery, TWF style, Dual Wielder feat, and then the DM just shrugs when the third magical greataxe drops and says, "that's what I rolled"?
Yes I think that is being a good DM and I will note the party got a Staff of Striking at level 2 that did not fit anyone's style.
I don't think it is particularly good game mastering to never curate the results of the dice. I know there are people in the hobby that insist this is the only virtuous way to play the game, but I don't know any game designers that would agree.
I think it is a lot more fun to drop the third Greataxe on my Ranger-Monk than it would be for her to find a Dragontooth dagger because I happen to use dex or simple weapons. I just think the latter is lame and not as fun.
Making what you find work is part of the game and building a versatile character (for example a Ranger that did not dump strength) should be rewarded.