new versions of OSX are free.
price is comparable to similarly specced PCs (where you can compare it...no alternative to 5K iMac, equivalent of graphics cards from Mac Pro can't be bought separately etc.).
apps on background stay if there is enough memory, otherwise they're swapped out, so who cares. I think they keep screenshot of last state cached, so it seems more responsive.
and you definitely can use linux on apple hardware.
out of warranty service costs is something I have no experience with, so can't comment on that.
// and no, I don't own any mac, I'm long time linux user...
there are other reasons why I don't like apple hw...no trackpoint on laptops, soldered/glued almost everything in laptops, outdated versions of core unix tools etc...