Turning a blank slate into an NPC is a serious pain in the ass with no upside. Hawke is a fully developed character and therefore easy for them to write for. The warden has no personality except in headcanon. Therefore, every single warden is different. No way to write for that. 90% of the appeal of the HoF is the self insert ability that a voiceless, personality less template character allows. Take that away by making an NPC with dialogue, personality, etc and you'll hear massive wailing.
Plus... you need a boatload of dialogue variations even if you got past that given how differently things could have turned out with respect to Leilana, the Warden's final state, and so on.
He certainly can't be the replacement for Stroud. Do you really think the playerbase would accept that their precious immaculate warden is on the run from the Grey Wardens? There's already people whining about how Virmire decisions are sucky and shouldn't be in games. Imagine if it was a choice between two former player characters? That's not going to have upside.
Not to mention if the Warden is King of Ferelden. He's sure not rampaging around Orlais in that situation.
Also, the fact is that the Warden isn't anything particularly irreplaceable. The HoF is not head and shoulders above others in personal ability. There's been plenty of other people on par with the warden in single combat.