And several extra seats on the aircraft since the refits. The crew are not the cause, but they could in some cases do more to help with the enforcement of 2nd piece under seats and stopping "dump and runners". Would help if they didn't use row 1/2 lockers either.

If compliance was improved through better enforcement, the problem would be mitigated. I witnessed this on a full A321 recently from TXL to LHR. The gate staff were tagging bags correctly (priority boarding was chaotic, but that's another story), and the CC were excellent at telling pax to put bags under seats. There was enough room for everyone's bags and at pushback the cc at the front of ET was even offering to put some more bags up from under seats. It is possible with the current allowances...