Except IGA gave them a 400 year blank slate (between LoI and CV3) along with 1999, so there's no need to close off the timeline.
They still need not to mess it up with unneccessary retcons. And besides, it obvious that Mathias wouldn't be present in this timeframe as Dracula. And since for many people "Castlevania = Dracula" then this timeframe is not an option for a game.
There is something that could be done with that, of course: for example you can play as non-Belmont character, but that once again will yield retcons or unneccessary complications to the already established timeline.
And one problem I have with LoS (aside from its abbreviation being too close to LoI and LoL), is the Combat Cross. Looks like a total Masamune ripoff.
That is such a trivial and minor thing of no importance, that I am not sure why anyone can have problems with something like that. By that margin people could have been pissed of that Simon Belmont of 90-s was a blatant Conan rip-off. And that's much more important than design of the weapon.
Any of them, being 3D or 2D.
Will not happen. Past is dead.
IGA will not work on Castlevania (most likely).

I don't get this reasoning. We already had self contained AU games before like Circle of the Moon, the N64 games and the Kid Dracula games. Despite their existence, it did not stop the main timeline from being continued. Why would it be any different with Lords? The only difference is that it's a self contained trilogy. I'm not saying I'm 100% convinced that they will return to the old timeline after LoS, but you're just being dismissive for no reason.
Because no-one have ever continued those timelines, and I am not sure how many regular gamers are aware that they are not part of the regular timeline. I have doubts that any developers will create continuation of any of those games on purpose. For that they should be big fans.
And as far as I am aware CV64 and COTM weren't advertised as "completely new take on the series / reboot". LOS was advertised like that and made a very loud statement about how its storyline separate from old timeline. Its something that certainly will leave impact on the next developers, Konami and the series as a whole.