Not really a back door but more of a front door. FTP is the basis of most road training and is the leading indicator of fitness. The famous ratio of 6.7 watts/kg to win the TdF is about FTP. According to my coach, FTP is very trainable but takes a long time. His example is filling a big swimming pool with a garden hose. When you train at FTP you are filling the pool. When you training at VO2 max you run a fire hose for a short period of time. However, the z5 efforts only work for a few weeks and then a break is required. Now some very good crit racers may have lower FTP but great abilities to sustain large numbers of accelerations at high power and sprint very well. If you want to ride crits, then the training is different but FTP is still the basis for the power zones and ultimately the TSS.

My FTP has been going up but very slowly. If that were not the case, we would all be much better than we are. So power training, much like HR training starts with threshold and one has to figure that number out. The best way is a test. Since I time trial and do long hill climbs, I get FTP tests all the time.

Also, most cyclists get dropped from group rides due to accelerations of 30 seconds at 20% over a power they can sustain for that period of time.