The thing is the Renault drivers who might qualify for a super license are rather... well they haven't shown the potential Leclerc, Norris and Russell did. Jack Aitken in his third season looks average (decent, capable of a F2 win or two a year but nothing special, and much of the grid are just as good, though he does have a super license) though Zhou has looked a bit more promising on occasion, though he has been a bit inconsistent. He deserves his second season though, so I wouldn't rule him out yet - lacks a super license though.
I'm not sure if Alonso is the right choice, but even if they qualified for a super license I wouldn't pick either of those two junior drivers at the moment. Personally I'd look to bring Hulkenberg back as a relatively cheap and reliable option, if he would want to return after being unceremoniously dumped. I think it's ludicrous he's not in F1 and I'm surprised he wasn't in the running for the second Ferrari seat.
If Alonso is rusty (and he should be) then Ocon needs to stand up and make the most of it - he'll only be 24, but in his fifth season next year. Ocon has been rated as one of the sports top young talents, but he's never truly shone, depending on how much you rate Perez. He's got every chance against Alonso. Vandoorne was similarly promising, but he only ever partnered world champions (Button and Alonso), whereas Ocon has taken small steps. If Ocon can't beat a returning Alonso then it will probably end his F1 career, but be deserved. If they're similar or he beats Alonso (if Alonso looks sharp) then it cements his status as a promising driver going forward.
I've always been of the belief that a driver should quit when he wants to or when a team no longer wants them. Alan Jones, Nigel Mansell and Alain Prost all went on too long, but if a driver is in good shape and decent enough then I've no problem. It's what I would do if I still enjoyed it and a team wanted me. I didn't think any worse of Schumacher returning after four years out - I thought of it as two F1 careers, one as one of the true greats, the other as a deserved swan song - it was just merciful he didn't race that notoriously difficult (and rubbish) Ferrari F60.