Hello!
Server load is indeed, and it has always been, one of the variables which are taken into calculation to determine the server rating and therefore recommend a server over another. Furthermore we have other inner variables which are monitored and are indicative of a server overload. In extreme cases yes, a server may be even closed to new connections due to overload, nothing new here. Such events are quite rare.
Where does this assumption come from? There's no evidence that OpenVPN users are slowing down WireGuard users. The problem is mainly on the OpenVPN client side, as users running OpenVPN on agnostic networks and/or on devices not supporting AES-NI might be unable to reach the performance which they would enjoy with WireGuard.
Again, whether or not prioritizing WireGuard over OpenVPN on the client side through our software default settings is not a problem due to OpenVPN stealing CPU time to WireGuard on server side (as incorrectly understood by @ms2738 too, we see now), as all servers have plenty of CPU available time for WireGuard, but a problem of customer satisfaction (in our previous message we mentioned the pros and cons which make the choice non trivial).
Kind regards