Out of curiosity: Could people who have really enjoyed playing fighters note what levels they played them to?
I played a battlemaster up to 14th level in one campaign that ended shortly before my current campaign in which I am still a 5th level bard. I have playtested every class in the PHB at every tier and participated is many non-campaign adventures of various levels; usually those are 3rd level but several were medium and high levels.
It's been difficult to play a campaign to 20th level in a year, and I managed on a bard previously, and I have an 18th level thief in an alternate game that doesn't run as often as the current game.
Given the expected time frame for a campaign of a school year, I imagine not a lot of players have played full campaigns in many classes yet and should be about the same one or two high level characters with regular play. I would expect many players did what I did and tested to see what they liked before getting committed to a campaign, and many simply continued with old favourites.
My favourite 5e classes are bards, rogues, rangers, fighters, and warlocks. Pretty much in that order.
I had a lot of fun playing fighters and could have fun with any class at any level. I do find most wizards, druids, and some cleric domains boring, berserkers aren't for really my flavour but I can do something with them and I know a couple guys who like them, sorcerer subclasses aren't my flavour but managing sorcery points was fun for me, and elemental monks seem to costly without enough magic choices while I really enjoyed open hand monks.
I am not sure why you might be concerned that players enjoy fighters. WotC did enough different versions of them during the playtest to come up with the released version. The public poll had them listed as one if the most popular classes and Mike Mearls commented that it was the most popular class iirc. The most popular classes in the public poll were rogues, wizards, fighters, rangers, paladins, clerics, and bards (just for reference).
I didn't like 4e fighters. The power system felt gamey on the class and seemed to limit my choices instead of expand them. I found fighters okay in earlier editions but always preferred spellcasters. This is the only edition where fighters are one if my preferred classes that I really enjoy so after over 30 years of playing to finally get to that point I think they did something right in that playtest and new edition. ;-)