I believe that IF they could make it until she was "elderly" he would stay with her through that.
But, I don't believe they would make it that long. Even if they could fix this nasty "loophole" with his soul. As mentioned before, Angel has some control issues and when things aren't going his way, he appears to take matters into his own hands and make decisions that involve other people without giving them a say in the matter. I don't think Buffy could ever handle that. She's very head-strong and VERY prone to liking to do the same, but never understanding when people leave her out of the loop or make decisions (even ones that DON'T involve her) without her having a say.
It may be why I'm a Spuffy fan. Spike was more of a "team player" and didn't seem to have much of a problem with including Buffy in most of his plans, even when he knew it meant she wouldn't be very proud of him. Angel doesn't do that. If and when he knows the people who love him won't approve, he sets out on his own. I think Bangel=death to one of both of them. Or at least a very ugly situation one day when Angel wants to handle something one way and Buffy prefers a different one.
YEA SPIKE!