When Finn gets into kamikaze mode, we see the ram start to tear his skimmer apart. It’s clear he won’t even make it, let alone blow the whole thing up.
Even if he did, there’s no doubt that the FO would find another way in. The goal was just to buy some time to make contact with their allies. They were fucked either way before Luke arrived.
You say these things as if they are certainties, yet they are anything but certain. Rose intercepts Finn’s speeder right before it enters the machine, and they’re crashed almost right beside it. It isn’t clear to me that he wouldn’t make it.
Similarly, it’s a plot point that there’s no other way into the base, at least until Rey uses space magic.