I want to say that it is not just the firing lines of B8 L42s, but also the absolutely insane amount of M56Ds marines can bring right now, which are very hard to counter in any head-on attack. They are so effective I have seen marines hold forever in the middle of caves while the entire hive was pushing with the queen, just by setting up cades with tons of M56Ds. In the past this would get countered by boilers that make use of the tight nature of such caves, but, surprise, boiler bombards are blocked by M56Ds for some reason. Just looking at the list of random stuff that blocks boiler bombards would massively improve xeno sieging potential.
Also crushers are indeed garbage now (all they do is stun and cause slowness, which isn't useful for sieges) and when defenders could siege effectively they were nerfed so hard that fortify isn't worth using anymore.