My two cents worth.
First, I am strongly opposed to totally eliminating the standby line for any ride.
But, I am far from convinced that this "test" is a sign that complete elimination of any standby lines is coming on a regular basis. I think that what they may be doing is trying to see if they can manage the distribution of FPs in a way that would maintain a steady flow of guests onto an attraction without creating an undesirably long wait for FP holders or wasted capacity. This could be part of the consideration of whether a "FP Only" track for TSMM would be feasible.
I have thought from the beginning that one possible feature of MM+/FP+ could eventually be a "virtual queue" system that would eliminate the standby lines of an hour or longer that frequently develop quickly at a couple of the most popular attractions, especially on very busy days. My idea is that the standby line could be allowed to reach a certain length to ensure that there will always be enough guests in line to keep the ride operating at maximum capacity. But, once the line reaches a certain length (such as 45-60 minutes) anyone wishing to go standby would be assigned a time at which they could return to the standby line. So, instead of waiting in that line for 90-120 minutes or more, the guest could do other things and then wait 45-60 minutes in the line later. By using
MDE and the magic bands, it should be possible to ensure that each guest can only have a space in one virtual queue at a time.
I think that some standby line is necessary to avoid wasted capacity on a ride, especially at the very beginning and very end of the day. Even if enough FPs are issued to cover capacity for the whole day, the ebb and flow of people returning (and no shows) will create peaks and gaps in the number of guests in the boarding area. Even the current test at TSMM recognizes that, as the reports have been that the standby line is open for a short time after the park opens and at least some times after that.
In the meantime, I'm willing to see where this goes before getting too worked up about it.