I think us stepping up in the FA Cup is actually a bit of a myth tbh. It sounds kind of ridiculous to say it but I’ll explain.
If you think back to all the FA cups we’ve won recently, we have had some real luck on the way.
In 2014 we almost lost the semi final to Wigan and required an almighty rescue against Hull in the final as we all know. We didn’t step up in those matches imo, we just won them in the end against inferior sides.
In 2015 we really only had to play United on the way to the final, where we slammed a relegated side.
Last season we got to the semi finals having beaten two non league sides and a Championship side in the process. In the semi finals we should’ve lost to City tbh because they hit the bar 3 times and had a goal wrongly disallowed. Theres no doubt that in the final they rose to the occasion against Chelski but if you cant get up for an FA Cup final youre probably a corpse anyway. I think a lot of them thought Wenger was off, and they themselves were off, and wanted to give a performance. I also think Chelsea had half gone on holiday already, which is obviously stupid from their end.
So a combination of lucky draws and lucky results have been matched by some top performances and weve won 3 cups recently. However, even if every performance had been top notch, the FA Cup is 6 ties to win. Its not exactly a marathon, and its clear as you said that Wenger cant do it for 38 matches anymore. I think these cup wins have been a godsend for Wenger because hes basically been getting new contracts out of them.