I wonder if CBS were to throw enough money at them, if Thomas/Bay would go for another season regardless of whether or not Segel were to return. Just speculating here, obviously, and I'd be 100% against trying to continue if Marshall had to be written out, but it doesn't look like it'd be that hard. How I Met Your Mother, at this point, seems to have become about Barney, Robin, and Ted. And to be honest, I can't remember a point where Marshall and Lily have felt so inconsequential to the bigger picture. It feels like their story is really over, and now they're there just because they're main characters. They're married, Marshall has his dream job as an environmental lawyer, and they have a child... so what more stories do any of them still really have to tell? Every goal that they had has been met. So while HIMYM would suffer, like any sitcom where a beloved main character leaves, it's not like you couldn't make a season's worth of episodes without Marshall involved (you know, assuming Ted, Robin, and Barney would have another season's worth of stories to tell and big developments to build to, which they don't). Obviously, that's probably not going to happen, it shouldn't happen, and I definitely don't want it to, but it's not like it'd be impossible.
In any case, I still think this season should be the last one, regardless of what happens. I just don't know what there could possibly be at this point to justify a 9th season to HIMYM. This show feels so tired, and the only big plots left to conclude are Barney/Robin and Ted meeting "the mother", both of which are pretty much set up to reach their conclusion at the season finale. I know you could probably have Ted and "the mother" dating in the next season, which I think hurts the purpose of the show (which is to show Ted's growth into being the person who marries his wife), and you could probably manufacture something else for the others to do, but... no thanks.
I really enjoyed tonight's episode. Lots of funny moments with everyone hiding around Barney's apartment (absolutely loved the bit where Robin kept finding secret places for books like "the Bro Code". Is it sad that I fell for it both times?), and obsessed Robin is more entertaining than normal Robin, as is her constant hatred of Patrice (the bit with the purse got my biggest laugh of the night). And the whole running gag of everyone taking Ted's stuff was hilarious.