I will be disappointed if he doesnt go out on loan next season for a few reasons. First, for his own development he needs game time at a high level. He is unlikely to get that here at the moment. Second, he is in competition with players in the three behind Kane - imo at the moment he is behind Eriksen, Lamela, Dembele, Alli, Son, Chadli and even Mason/Caroll for a start in those positions. Third, at this stage of the season where every game is virtually a cup final, he should not start in front of more experienced players who have shown that they have already done it this season. Fourth, he has now had quite a few minutes on the pitch and yet still does not have a goal or an assist to his credit - so if we are talking about those deserving to start - all the aforementioned are goalscorers and assisters this season and therefore logically should be ahead of him imo. That is not to say he is a bad player or that I don't rate him, just that imo he is not ready yet and is behind better players in his development.
Your examples of when Poch brought him on in games simply means he opted for a more attacking option to try and win games (or score more) than the more defensive options available to him. Not at all unreasonable and I have already said that Onomah represents an option off the bench but not a starting option at this stage of the season. Poch is simply using the squad that is available to him in the best way possible and as circumstances dictate. We were fairly thin on the ground with injuries meaning some options are not available to him at this time and rotation is required to guard against burn out.
My final reason for being disappointed if he doesn't go out on loan next season is that it will again mean we have failed to strengthen our squad in key positions. We have too many AMs at the moment and with CL beckoning (hopefully) I hope we will make key signings in key positions that will better equip us to handle the rigours of CL football.