I'm not saying that's the right thing to do. Just that it probably is what it is.
Look, I've seen every Underworld movie except the last one, and while I vaguely didn't NOT enjoy them, here's my take-home from each one: Bunch of people in black, book-loads of backstory I don't care about because all you have to remember is that vampires hate werewolves, blue filters, gray cityscapes probably in eastern Europe, Kate Beckinsale doing everything short of getting naked because her director husband is protective and is only cool with her making out with other guys in tight leather, blah blah blah Lycans and Death Dealers, cliffhanger non-ending. Hell, even the Hollywood Reporter article announcing the reboot can't remember if the main character's name was "Selena" or "Selene." (They may have fixed that by the time you read this.)
The last Underworld movie came out two years ago. Realistically, what was different then?
Kate Beckinsale was 39. And while I don't have any particular loyalty to this franchise, it seems kinda shitty not to keep her around now that she's 41 (a man her age would not be put out to pasture). But they're not talking "sequel" - they're talking "reboot." Again, I don't know that it's the age thing - but considering nobody else is clamoring to buy back the rights to this series as far as I know, I don't see any other reason to start over. Does anybody really care about some element of this series that isn't either Beckinsale...or another actor that's been killed off already, like Bill Nighy?