You don’t need to do an assessment unless you have no idea / or no interest in guiding your training.
AT does not need FTP to make you fitter. It uses every workout you do as an assessment. If you have low PL numbers, then you are not really doing the workouts in the energy system that is claimed. Or, maybe more precisely, it is on the fringes of the energy systems. Same for if you have high PL numbers.
If low PL number workouts are hard in the areas you care about, I would manually bump FTP input down. If high PL number workouts are moderate, I would manually bump up a couple percent. If they are easy, I’d do that and pick some stretch workouts following to recalibrate the system.
In a rate of rapid fitness gains and especially in the absence of knowledge of your abilities, ramp is a great idea. Once you get in that shallow rate of fitness increase, error in the ramp test starts to overlap with fitness increases and can be counter-productive. It’s very probable to get a static ramp score or even slightly decreased ramp score after a productive training block. Objectively that’s not gonna matter much unless you’re sitting at the max range of PL for energy systems you care about. But mentally that’s a hard pill to swallow unless you understand the test is limited. And once you understand that, well, it doesn’t make as much sense to do it. Similar things can be said if you over score your actual FTP, but AT kinda fixes that by telling us we have a great FTP but giving us workouts that we can manage even though we don’t.
Related to all that, finding threshold level 5 workouts “hard” ( I.e., not very hard or all out) means one is in that range where your FTP input is fairly close to accurate. For a person hyper focused on FTP, that may not be true - that person very well have an FTP that they can hold for longer than 40 min, or they just have a really precise assessment for it because they ride there a lot. But for the average Joe, it’s likely close enough. For me, that means I’m level 10+ in everything lower than threshold. So most production in fitness comes from really long SS intervals, plus threshold and VO2 labeled workouts.
If I raised my FTP input setting, most of my fitness gains (that I wish to target) would come from a wider sweep of the sweetspot catalog of workouts, which will be hitting that threshold end of the zone more, and threshold, which will be trending towards VO2 more. The VO2 labeled workouts will be the easier versions of the short on/offs that are mentally boosting, but I am a bit skeptical do much for increasing my fitness.
In either scenario I’m still targeting the same energy systems, they’re just being pulled from differently tagged workout bins. There are no hard boundaries on any of these things and in the end, the only thing that matters is overloading your system with proper recovery. And AT is a nicely designed approach for systematic progression towards that if you don’t worry too much about how the energy systems are labeled.