I doubt DOA6 will be a completely rebooted overhaul you think it will be, just look at the gap between DOA3(og xbox) and DOA4(360) or even NG:Black compared to NGII they were a minor upgrade despite the later sequels being built from the ground up on a newer gen console. i expect the same for DOA6 compared to LR. accept with higher texture resolutions, AA, and bigger more interactive environments slapped on.
Wait, you are talking about the games having the same art style. Yes, the art style stayed the same, but the games were still completely new and all graphical assets (character models, stages, textures...) were new. These character models superficially look the same, but they are still different character models, which take time and money to develop:
If you think, Team Ninja will get away with using the same character models as well as the same stages as DOA5 for DOA6, just with higher textures resolutions...lol. That is actually not how video game developing and marketing works...
Better graphics do sell games! I know, there are purists, annoyed with all graphical talk and dick measuring, but it is a fact, that good graphics sell games. It's what you can convey the easiest to the potential customer in trailers or with screens. Especially Team Ninja was always famous for upping their game regarding graphics and they will again.
At TGS we got a good glimpse, of how DOA6 characters will probably look like with NIOH's protagonist, William:
It's the same art style as DOA5 and he looks like Bayman wearing Brad's wig, but this character model features at least double, probably even triple (if we are talking about male characters) the polygons, animation bones, texture quality and shaders of DOA5 models. Again, a model like this is very expensive and time consuming. You won't see 35 of these in DOA6 at release.