Over-under and V02 max workouts will increase your threshold.
So even if you never ride over threshold, 90% of a 250w threshold is still higher than 90% of a 200w threshold.
You can train to maintain higher percentages of FTP for longer durations, but I suspect that FTP gains will come easier and with more variety/fun.
Look at the plans for long distance steady state athletes like “sustained power build” or any of the “full distance triathlon” plans and you will find plenty of V02 max work. Yet these are aimed at athletes who, when racing, won’t be going over threshold if they can help it