Some users have fun in keeping or even collecting old aging players for whatever role playing reason. And I see nothing wrong with that, let them play the way they choose to, they do no harm doing that.
Then there is a good point about SC fodder.
Last but not least, there may be situations when they are actually good enough to be the best option. Even on NT level. First thing first - they don't start losing their skills before their turn to 33 years old, so any suggestion to retire everyone older than 32 is just wrong. Then there can be nations small enough (number of users of given countries may vary wildly over time) to have limited pool of players trained to their potential (or trained at all) to such measure that even aging player can be a star, rotation player, bench player, injury replacement for their NT for many years. If you happen to lose users to the point that every domestic team matters or even to 0 users, then every random older player of that nation floating around the world of buzzerbeater counts. If there are none to replace him (yet), he gets his spot by default. The thing whether there is a point to have such a limited national team is another one but for now such team owuld go nowhere and there is no legitimate reason to hinder their play even more.
In addition, such oldies player would still get experience thus getting better in something (whatever it is that experience influences).