If you think he's legs are gone, you can't be asking why he isn't playing as a #10. Even tighter spaces and more aggressive defending, plus the need for strong feet that can give you good balance and a burst of acceleration. There's a reason why most ageing players who start as more advanced midfielders with good vision and sense of space transition to deeper roles and not vice versa.
Eriksen still possesses qualities a team could use on a football pitches. He can use both feet, he reads the game well, and he has a good passing range. He showed it last season and this season too in glimpses. Of course, he comes with his limitations and his age means that his playing time needs to be conditioned. The latter will be fixed when Mount and Amrabat (and to a lesser extent Mainoo) become available. As for the former, if United persist with this kamikaze football style, we won't be able to get much out of, not only Eriksen, but any decent baller we put in there.
It all comes down to what kind of football you want to play.
