Arguments like Fighters can improvise, can get magic items or feats and other such BS read to me as "I want Fighters to suck and I know that I have no good reason".
That's a bit extreme.
Fighters don't "suck". They really don't. And it does behoove everyone to start from the position that fighters as written, do work most of the time. Again, since most campaigns tend to wrap up in single digit levels, a lot of games never really see the problem. At single digit levels, fighters don't really fall behind too much. And any falling behind that does happen tends to get glossed over by the party simply either not doing X or doing something to help the fighter up. So, the fighter gets those boots of flying by default, that sort of thing. The fighter is always the first in line for any buffing spells. Every magic weapon goes to the fighter first and the DM always makes sure that there's a magic weapon to be found. Heck, this was built right into the earlier editions because something like a quarter of all magic items (and you did get rather a lot of them) were magic weapons.
And in the single digit levels, the disparity isn't all that noticeable because the casters are still using their lower level slots on stuff that doesn't overshadow other characters.
It's mostly in the double digits where it becomes a more pronounced problem. The baddies just have so many ways to shut the fighter down - either by having different movement types, or the fact that adventures at higher levels stop being in mundane locations as well. That fighter is pretty pooched when it comes to any adventure that occurs underwater. You want to do Spelljammer? You MUST have a caster. Your 20th level fighter can never fly a Spelljammer ship. The adventures themselves change in nature and that change really highlights the disparities between casters and non-casters.
And then at high levels, the casters don't have to use their lower level slots on combat anymore and have slots to spare. Suddenly all those sort of niche spells that are so powerful, but, so limited in use, start getting pulled out. 6th level casters probably aren't using Locate Object spells, for example. They just don't have enough slots. But a 16th level caster? They've got tons of slots to spare. Detect Thoughts is a minor cost and a huge effect. On and on.
And that's where the problem really lies. Your fighter will NEVER be as good as my wizard at interrogation. Doesn't matter that you've got Batman levels of skills in Insight, my Detect Thoughts spell will always be better.