Many professional and amateurs work on fueling their efforts so that their fitness parameters (TP/FTP, HIE and PP) don’t erode over longer rides. Using a fixed signature is what causes MPA to stay elevated and predicting the influence of this fatigue on your signature is what would be required for MPA to show a greater decrease over longer rides. As a thought experiment, what would your FTP be after doing a 20 minute FTP test? After 5 hours of riding?
We could reduce your TP/HIE/PP as you perform and thus increase XSS for longer rides and thus assign more fatigue to your rides but a dynamic signature isn’t a simple problem. Having said that, a model for dynamic signature values is in the works with Xert 2.0.
Every other system, for the most part, is trying to pinpoint FTP, generally by a regression or AI on months worth of data. There is really no thought or possibility of adjusting FTP (and therefore TSS) over the course of a longer rides. That’s a long way away. Getting a daily FTP is also beyond their current state.
I would suggest you first think about your fueling and why you’re fading as much as you are during longer rides. This can be trained and improved.
Good luck with your training.