I think Mike's making a big mistake. GDT's a terrific creator, and while someone else might make a new Hellboy movie or TV series or whatever that's closer to Mike's vision I severely doubt that it would be as... well, good.
I also think it's short-sighted, because leaving the trilogy unfinished and coming off Ron's beloved performance could poison the well for a reboot. If a non-GDT, non-Perlman reboot comes out in a few years people will be predisposed against it because of the goodwill the previous films have.