Thats just false.
TF2 has way more variety, as each character has multiple different weapons. that can be combined in any way, and played in any style. Overwatch has tank, dps, support. Yes, all of those characters have different mechanics, but they play largely the same.
Some examples for TF2. You want to play as a soldier who demolished people with their rocket launcher, and sends a giant laser blast into crowds of enemies that catches them on fire? Well, you can, but with that same soldier you can propel yourself into the sky, and across the map, and whack someone with a shovel and insta-kill them if you are good enough. Again, with that SAME SOLDIER, you can blow a horn that makes your teammates faster and leech life, or tankier, or boost their weapons to do more damage. You can also build that soldier to be a tank that can consistently leech life off players while simultaneously healing passively after not taking damage for a while. Thats just a few things you can do with a SINGLE character in TF2. This doesn't take into account the varied playstyles and different mechanics for the other eight characters. Heck, even with one of the most linear characters (sniper), you can get items that drastically change how he is played. Like a jar of pee that can extinguish burning teammates, reveal cloaked spies, or increase the damage dealt to enemies, or a bow which takes him from the backline, to the frontline, and allows him to be more aggressive.
With overwatch, most of their supports, tanks, and dps play near exactly the same. You don't get nearly as much variety between playstyles with characters like mccree, soldier, widowmaker, ashe. With tanks you get even less, you eat damage, put up a shield, and protect your team. Support is just as boring as well, heal your team and occasionally buff them here and there.
Also TF2 being old is really a bonus when you think about it. It's over a decade old, still has a consistent playerbase, while overwatch is beginning to die out. Thats really a plus towards TF2, and a negative towards overwatch.
You can also point out how despite the fact that TF2 has been neglected and rarely updated (unlike overwatch), its still consistently fun to play. Overwatch has dev support, but that same support is backfiring on the game and hurting it even more.