I'm not too worried about the new respawn timer. I do see that the maximum cumulative respawn timer of 120 seconds *might* be a little long, but its not too far off the mark. Cryptic can easily to tune it down to 90 seconds if too many STFs fail due to people waiting to respawn.
I hate bringing out my past but I've been doing much tougher content in EQ a decade ago. Even with this new respawn timer change, I can easily say that STFs are still much easier to than the most encounters I have faced in EQ. As every player have easy access to some form of shield regen and hull repair, the healer has a much easier time and can even focus on offensive attacks rather than having to focus on keeping the Main Tank alive. Compared to EQ where the group is essentially dead if the healer misses a heal or two on the MT, STF groups have a very large leeway to recover from any mistakes the group makes.
I forsee that the way groups are organized will change. We will see much less of random pugs and chances are a new NormalSTF channel would be created so players can organize themselves into groups of MT + Healer + DPS before starting a STF. Expect to see more specific "CSE LF Tank", "KASE LF Healer", "ISE LF2M DPS" calls rather than a general "LF2M CSE" call.