Valve plans to re-release every popular mod currently using the Half-Life engine and will see that modders have a full toolkit for converting their mods from HL to HL2.
The implications of this statement are truly staggering IMO (Think Counter-Strike.) If they pull this off Valve can totally own purely from the longevity and popularity HL2 will achieve in the gaming community.
Having said that, I'm still certain that plenty of top commercial outfits will pick up the DOOM engine. Especially those who already have a relationship with id and are planning on making sequels to well-established Q3 titles. What THEY are able to do with the D3 engine, well I dunno, the possibilities are mind-boggling (A MoH game in the D3 engine anyone?)
I really was expecting D3 somewhere around the 10th anniversary of the Original though. Oh well no big deal.
"Nothing livens up a robotic hymn of doom more than an amazing pair of jugs." - Brak