I've been on the receiving end of the Sephiroth Dive (and shamefully more than once), but I disagree on the point presented.

I would not consider having a ninja to transfer/lower enmity or using a warrior to frontload a bunch of enmity to be a gimmick or flaw in design. That a healer generates aggro and the party being forced to deal with it in some manner is an appropriate cost/risk as well as a reasonable puzzle piece in the problem that an encounter entails. Though it is unfortunate that paladins have weak enmity generation in their main combo, I am of the opinion that the healers have sufficient tools to deal with the problem.

The WHMs have it easiest with Shroud, and often only have themselves to blame. I have personally not seen a paired SCH top the aggro list on bosses from just healing, which leaves the AST. For those I am of the firm belief that good timing of LA can solve a lot of problems. A tank dying and passing on top enmity to healers is a case I do not consider relevant to this discussion - that is a failure solely in the hands of the players, not the system.

Aside from the T9 surprise, I don't think any other boss in the game opens (as in "cannot be targeted before") a phase with a massive targeted spike. If future bosses follow this pattern, then there is plenty of time for tanks to get their shit together and grab aggro.