The automatic transition would not cause additional harm in the compromise scenario and would in fact marginally help. A client which has not got an out-of-band

The automatic transition would not cause additional harm in the compromise scenario and would in fact marginally help. A client which has not got an out-of-band revocation/update of the host key is subject to complete mitm'ing and impersonation either way: tricking it into accepting a new fraudulent host key doesn't actually make this scenario worse, it only means the first attacker locks out his competition :). (An out-of-band key update still fixes the client.) OTOH, if the client happens to connect to the authentic host first, then it is saved. So, no net harm, marginal net benefit.