You're right, this is the first time a Rider hasn't had a protective field while performing their henshin in a very long time.The Swordsmen acted pretty helpless to deal with Durandal's teleportation ability, but we were shown pretty clearly that he has to separate and reconnect his weapon in order to trigger it, so they should've caught onto that and attacked him at that moment, to keep him from completing the gesture. Kamen Rider has generally been pretty good at avoiding the implausibility of an adversary just standing there waiting while a fighter completed a transformation or weapon activation or whatever. The Showa Riders tended to jump up somewhere high so they'd have a free moment to do their henshin poses, and the majority of the Heisei Riders, at least since Blade, have had energy fields or constructs around them that repelled attacks (or sometimes even inflicted attacks by themselves) until the henshin was complete. But I don't think Saber has that -- they just have the Swordsmen stand there holding up the books while they recite their long speeches, and so on.