I think the album Killers overall is more punk than even their first album. And this was 1981, which is definitely after the punk explosion. Punk had already run its course by 1980. Punk's big years were '76 through '79, particularly in England.
Perhaps the parting with Paul had something to do with Steve seeing the direction music was taking at the time. So I wouldn't necessarily say that early punk elements were forced on Maiden. If anything, they eschewed them (and their punk frontman) in order to keep up with changing times. Steve's denials of ever having been punk only came up after the fact, around '82 and after. As far as I know, Maiden never denied it during the period in question.
As far as where are the punk bands now - that is a bit of an unfair question. You can ask the same of the NWOBHM bands too. Or ANY band that was around more than 30 years ago. Where are Tygers of Pan Tang now?
Many punk bands went on to be pretty huge through out the '80s and '90s - Siouxsie & the Banshees, P.I.L. (Johnny Rotten's post-Pistols outfit), New Order (formerly Joy Division), the Cure, etc. Punk really didn't die, they just learned how to play and it evolved into New Wave and goth.
The idea that punk and NWOBHM were even all that different needs to be examined. If you look at what metal was prior to NWOBHM, it was heavily blues based, from Deep Purple, to Black Sabbath, to Led Zeppelin to Aerosmith. Punk changed all of that, particularly in the UK, and the result was NWOBHM. It even has "New Wave" in the name. British metal shook off the blues and took on a punk-rock ethic. There is probably no metal band that exemplifies that better than Maiden.