All he'd have to do is change some characters' names, not change the entire plot. We don't even know the entire plot.
Names are half of Castlevania. You change to many names and it becomes John Randeer the vampire hunter and his sidekick Bobby Grey, who is hunting Dalkas Stockovich. It stops being Castlevania and starts being a cheap ripoff.
Besides, I like Belmonts better than Morrises, and I know a lot of other people do too.
The only thing that's known/established about the 300 years between LoI & CVIII is that the Belmonts were vampire hunters & were feared by the people. Dunno about you but I could think of hundreds of ways developers can tell stories without stepping on anyone else's toes, without any of those "rules" getting in the way. And there can certainly be plots after Soma's time. Alucard does hint that a new Dark Lord would eventually emerge, so they can set a game 100 years after 2036. Does that mean the aesthetic has to change to accomodate futuristic hoverboards? No, because those stories are left wide open for anyone to decide what happens. You're making it seem like everything about the CV universe is so stifling for developers, when it really isn't if you think about it.
You start changing Belmonts to other vampires and the villains to non-Draculas, it's a slippery slope. Eventually you'll have a wandering Japanese swordsman with distant blood relation to the Belmont clan defeating demons. Whoops, now you have Devil May Cry. Characters and gameplay are what make a series a series, not a loosely tied together continuity.
The Castlevania universe isn't stifling, but the canon is a tad.
And really, how many origin stories do we really need? Once CV turns into a series of reboot after reboot, just because the developers don't want to deal with canon, then...
Then we have a bunch of reboots. Every Legend of Zelda starting with Link to the Past is practically a reboot and nobody seems to have a problem with that. Origin stories are exciting.
Konami has done it with Metal Gear, so the "nobody in the history of video games does that" comment is nonsense.. I don't see a problem with them doing it with CV.
Exactly which Metal Gear box has "this is a stand alone story" on it?
The difference is, Zelda has been doing it since day 1, Castlevania hasn't. For them to suddenly switch everything up would be pretty stupid and unnecessary. It's like they'd be trying to appeal to the Zelda fans, like LoS is trying to get the DMC fans. Just because Nintendo made it "work" for Zelda, doesn't mean Konami can or should make it work for CV.
Castlevania is borrowing a lot these days, lets not add "Zelda" to the list.
Why would it be stupid and unnecessary? They wouldn't be trying to appeal to Zelda fans. Putting the next Belmont in a green cap and skirt would be appealing for the Zelda fans.
The only thing LoS has in common with DMC is the combat. Does that mean SotN was trying to get the Metroid fans?
Borrowing a story telling method =/= borrowing from Zelda. Zelda isn't the only game series with a collection of unrelated plots, it's just a notable one.