Has anyone pointed out that the original and still intentional design of fishing is not to be a collectables thing, not to be something players just do and move on with, not even meant to be a place for players to show off their 'skill'?

It is - instead - designed to be a part of the game that intentionally slows the player down. It has, and continues to be, designed in such a way that the player has to stop and appreciate the environment and the world. "[Fishing] is a way for the player to communicate with the environment of Eorzea" is the quote from YoshiP around the launch of 2.0, I believe. You can even look at the fishing log to see how fishing and the world itself are intertwined.

Now, to your point on catching Lv50 fish in 90 BiS, and claiming "what's the point if it's not making it easier to catch?" The thing is, it does make it easier.

At Lv50, when the fish would be 'on content' - you have barely any skills. Fish would be significantly more difficult to catch. Even if you're, Lv90 in Lv50 gear, you won't have enough GP to utilize some of the recent skills. Shonisaurus, over in Witchdrop - when it was released back in 2.5 or something, you had only Mooch to use, which means (at the time) you had to get lucky with your bites and hope for multiple HQ (now large-sized) fish in a row to even had a chance.

Come Heavensward and the level cap of 60, Fisher's got Patience I and II (P1 was originally a Lv50+ skill!), meaning you didn't have to hope for luck to get your HQ fish to mooch; a HUGE nerf to the fish. Add in some new baits and in just one expansion, it went from being one of the hardest fish to catch to one of the easier ones.

Then Stormblood, we get Double Hook. All those Heavensward fish like Charibenet or Sea Butterfly that required multiple fish to trigger Fisher's Intuition? All nerfed because now you could catch more than one at a time, meaning more intuitions per window, and more chances to catch the fish.

Jump ahead to Shadowbringers and we get Identical Cast and Surface Slap - the latter being the most powerful skill in a fishers arsenal. Literally able to affect the chance of a fish biting. Unconditional, a Stormblood Legendary fish in Azim Steppe overnight became one of the most straightforward fish to catch. Use Stonefly Larva, catch a Nhaama's boon, and Slap it, and the next fish that'll bite in the window will be Unconditional, 100% of the time. Will it stick? Not as often, but it went from being "it'll take me multiple windows to catch" to "I can probably catch multiple in a single window." ... and this is before we even talk about the new Fish Eyes change.