You're joking, right? Halo is a huge selling point for Xbox. The biggest actually, despite there being a hundred other multi platform FPS. Without the original Halo, Xbox One wouldn't even exist because it kept the original Xbox from going under.
Evil within, dying light, etc. are bad comparisons. Is TLOU technically a post apocalyptic adventure game? Yes. But that's not why people buy it. They buy it because its real genre is Naughty Dog. Which basically means unparalleled linear drama, character and storytelling. Players would argue (and obviously it's subjective) that experiences like; TLOU or Halo or Uncharted or God of war or Gears or other system sellers are unique. They'd argue that you CANT get that same experience anywhere else even if there are other games in that genre. So they buy that console for that exclusive. Or at least that's a big reason why. Plus MLBTS is multiplatform (ps4, PS3, Vita) and will be for a long time so where's the system selling in that.
Sure MLB:TS was the only baseball sim last year but it's not like it did gangbusters for their sales. They did about the same amount of sales that they usually do. Is The Show a big reason why some people bought a PS4? Sure, I believe that but I also believe that group is very small. I think the audience for baseball games and baseball in general is very small compared to what it was. That's a big reason why MLB 2k (or all the other dozens of baseball franchises) went under. The market's not really big enough for both games (and SCEA just made a better product). Even if every person who buys The Show bought a PS4 mostly for that game, that's like .5-.8 million people and that's pretty much it for the lifetime of the console because it's not like a new batch of people are picking up the game every year. It's mostly the same .5-.8 million people every year. No way for us to know what games sell more consoles but I think it's perhaps telling that TLOU is bundled with PS4s and MLBTS will likely never have that honor.
Really the only argument I need against yours about genre similar games being hugely detrimental to system seller status is Destiny. Super generic, many alternatives to it, multiplatform, multigenerational (360, PS3), middling reviews and had extremely limited PS4 exclusive content and yet it more than tripled PS4's monthly sales in the month of September. Mostly because of advertising (and the Bungie element. SEE HALO).
None of it really matters though because almost no one buys a system for single franchise that would be somewhat ridiculous financially. Even Nintendo has a few franchises that people use as a rationale. Manly people accumulate reasons. A list of pros and cons and some games tip the pros more than others or are the final straw to finally making the purchase.