It's not so simple. Fighters need trained ground crews to operate and pilots need to be trained in one. From what I have heard, Ukrainian pilots are currently training to fight in F-15s and F-16s. Training the Ukrainian Air Force to operate on yet another different fighter is going to take even more time.

They really wanted Bakhmut so bad that they're willing to throw all their best chips in this gamble. However, unlike Severodonetsk and Lysychansk, this time they're facing fortified positions that has been in place since 2014 and with an Ukrainian military far better equipped than they were in June. Even if they did capture Bakhmut, they will be so drained that by spring, the Ukrainians will steamroll them in the eastern oblasts.