There is data available via Google for track athletes , sorry can't provide a link as I'm using my PS and I'm an old technophobe , how that translates to WRs is up for debate
I also seem to remember Dan from FBG saying he'd seen team data regarding James Jones from GB in an old thread that showed no significant speed regression until his last season
Not really disagreeing over your opinion , more it's a matter of degree tbh , it's obvious all athletes do decline , probably from around 25 onwards , but I believe recent Madden ratings have over done that
Having said that even applying a totally accurate regression arc universally wouldn't account for players individual situations , just provide a guide, so this type of discussion will never really go away