Putting aside his fitness because that's something he can do very little about.
He's actually been quite effective when he has played, indicating that he's very handy to keep around as a squad player.
He's had 69 starts and 42 appearances in the league, contributing 10 goals and 21 assists, as you said. In total, this means he's had 31 direct goal contributions in 111 games, which is slightly less than 1 every 3 games.
Given the number of sub appearances in that, that doesn't actually tell the whole story. Squawka has him down for 6435 minutes for Spurs in the PL. That means he averages a direct goal contribution every 207 minutes, so not quite 1 in every 2 games.
When you consider the disruptions he's had in his career and lack of continuity and momentum, I would suggest this is excellent for someone who is currently considered a squad player. In fact, I would argue that to do this in spite of his injuries is all the more impressive.
There's also a whole range of intanglibles to consider, such as his effectiveness in Pochettino's system, movement, defensive work and work rate.
So, why is that not good enough? Is it because he cots 30m or because you mistakenly believe that he was supposed to replace Bale? 30m is hardly anything these days, and as for the latter, that's a problem with your expectations then, not with what he's actually produced.
When you look at the objective reality, he more than justifies his place in the squad, and any other judgement is just not based on reality, just blind bias or preconceived ideas of what he should be.
