if it never reaches the 55% threshold then activation stalls

No, that's not what the BIP or the activation client code says.

Standard BIP9 uses a time-based timeout that transitions to FAILED if the threshold is not reached. This deployment sets timeout to NO_TIMEOUT and instead uses a BIP8-like, height-based max_activation_height. The deployment transitions to LOCKED_IN at height 963648 (one retarget period before max_activation_height), then to ACTIVE at height 965664.

Similar to BIP8, this deployment enforces mandatory signaling during the retarget period immediately before mandatory lock-in (blocks 961632 to 963647; lock-in happens no later than block 963648). During this window, blocks that do not signal bit 4 are rejected as invalid. Mandatory signaling ends once the deployment reaches the LOCKED_IN state.