The key here is how is T set. If T is fixed then 2T becomes the hard limit and the problem remains. If T is set based on an some average of previously mined blocks then this may address the problem
thisactually, just use twice the average blocksize of the last two weeks.
And you don't really need any of this complicated system.
1. Floating block limits have their own set of problems, and may result in a scenario where there is no effective limit at all.
2. Even if you do place a floating block limit, it doesn't really relieve of the need for an elastic system. Whatever the block limit is, tx demand can approach it and then we have a crash landing scenario. We need a system that gracefully degrades when approaching the limit, whatever it is.