"The only dealbreaker I can see is that the T5 is a tail-clicky instead of a side-clicky."
I think you kinda answered your own question. There is no other EDC that offers the SC60's combination of output, UI, and runtime in this size. It is unique in this regard. So far as AAs v/s 18650s I guess it's just a matter of opinion...but my opinion is that the AA platform (while convenient) simply doesn't come close to the capacity of an 18650 which to me is really not that much larger.
The SC600 certainly doesn't seem like a direct replacement for the SC60 since it is longer, much wider, and far heavier than the SC60. Two different animals, IMO.
Don't get me wrong, I can see why people love the SC60. Nice as my SC600 is, the larger head means you just can't EDC it (then again, I felt my LD10 was a bit big for pocket carry, so I don't think the SC60 would have persuaded me either, but that's clearly just personal preference).
However, I don't think what you said about size really stacks up. For example, based on current specs the difference in length between T5 and the SC60 will be greater than between the SC60 and the SC600, and the difference in weights will probably be similarly proportionate. Now the capacity of 18650 is definitely a major plus, but when you look at form factor and performance, there's no getting around the fact that compared with ZebraLight's 2011/2012 lights, the SC60 is technologically dated. What's more, the recent rush of last orders aside, it probably wasn't selling that well (as most of the kewl dudes here at CPF have already made that wise investment).
To be absolutely clear, I totally understand how the unique size/UI/output/chemistry combination made this an attractive option for many. But when I look at ZL's upcoming lights, and what they have to offer, I can understand why they're ending production.