Technically, if you wanted to acquire it legally, you could buy a used Wii that happens to have it installed (by someone who also acquired it legally, back when the Wii Shop channel was still alive). It's not really any different from buying a physical copy of a retro game. Either way, the publisher isn't seeing a cent of your money, only the reseller. It's just a lot more cumbersome to buy an entire system to get access to a game, and it'd be like trying to a find a needle in a haystack searching for one with all the games you want installed.
I sincerely hope Konami will do a re-release of the Rebirth games someday, even though I do have Castlevania and Gradius on my Wii already. I've actually been playing Castlevania Rebirth for the first time just lately. It's not among the best of the best of the classic-style Castlevania games, but it's still really solid. Puts up a decent challenge on the default settings too, in my opinion. I still haven't beaten Death, but I'm getting good enough at the stage before him that I'll probably reach Drac soon.