Its not selling yourself short though to build your posterior chain so you can focus more on your own strength rather than thinking the belt is the key to your set of 10 at 405. Have you tried this squat without the belt? Focusing on proper form and staying tight through the squat with a big belly of air? What about all the other exercises that focus on the posterior chain...Good mornings, hypers, pull-thrus, glute-ham, zerchers...etc, are any of these a focus of your workout?
All just questions and food for thought. Your statement about atrophy is not true. If you work your posterior chain you could most likely progress to 405 for 10 without a belt in a short amount of time. However, just because you aren't squatting 405 all the time does not mean your muscle tissue will diminish (unless squat is the only exercise you perform for legs - which I can't imagine is so. ) Something else to consider, muscle tissue enjoys the stimulus, but it also enjoys symmetry and proportion. For example, if you had injured one side (leg) and you work the other leg frequently - the body will compensate and you won't lose as much mass on the non-worked side. Also, nutrition plays a part as well. You can do all the hard work lifting, but without the proper amount of cals nothing changes.
...it just sounded so much in your statement as there was a dependence on the belt - I just wondered. No knocking you - just conversing.