Being able to adjust microphone positioning is very important for creating the best possible recordings.
That being said, being able to essentially flip the order in which two cardioid micro capsules are used in a 90° pattern with resulting minuscule adjustment of the microphone distance is mostly silly. Other angles than 90° don't make a lot of sense if you still want to record stereo.
The principal problem with microphone adjustment with handheld recorders is that everything is in a single unit. For best positioning, you need detachable microphones. Of course you also need microphones with a size and/or principle that delivers a somewhat reliable polar pattern over a sufficiently large range of frequencies. Small-size electret condenser capsules without much leeway for an acoustically significant encasing always are a bit problematic.
The TR-100 has two microphone pairs, one cardioid, one omnidirectional. The stated goal of the microphone adjustment on the TR-60 of being able to capture ambient sound is much better served by switching to omnidirectional microphone capsules, and with a four-channel recording, you can blend over as needed for your mixture of ambient and directed sound.
Also noteworthy is that electret condenser capsule cables and their soldering joints are thin and fragile. Overuse of the swiveling action may cause them to deteriorate as they will get bent around each time (for microphone levels, any kind of swiping contact would be prohibitive for the sound quality, so this will work by bending cables back and forth).
So all-in-all I'd say that the overall size factor is a much more limiting constraint, and that the overall set of 4 capsules with two different polar patterns should more than offset the swiveling action of the adjustable set.
I suppose that is the reasoning behind what I consider a sensible design decision. Whether the market appreciates that reasoning when being deprived of the satisfaction derived from mucking about with mechanical gizmos is a different question, of course.