Very harsh on both of them IMO. Only in Premier League Kane (born in '93) scored 21 goals in 13/14, 25 in 14/15 and now 14 in 20 games of this season - these are not stats of a "decent Premier League player", he's one of the best strikers in the league. I definitely think he is good enough to be a starter in a title winning team - it won't be Spurs, be he would do just fine in United or Chelsea (personally I'd take Kane over Costa comfortably). Not a world class (yet?), wonderful goalscorer and a very good player.
Alli is a year older than Rashford and a year younger than Martial. In last two seasons he has 57 PL apps, 21 goals and 12 assists. Martial and Rashford combined got total of 22 goals and 11 assists. I dislike Spurs with passion, but looking at Dele's stats and game I have no doubt he is one of the best prospects in the league, he has everything to be the best English midfielder of his generation. Next summer (2018) he could be off to a top team, I think.