Isn't ENB a Graphics enhancer?... yes it is!! so you should never trade graphics enhancement for performance. If people don't have enough power to use fixed TAA then simply don't. Go buy new hardware as this is a graphics enhancer, and as such, should not trade graphics quality for performance, ever (within reason).
If that were true then things like FXAA, SMAA, TAA, and tons of other graphics techniques wouldn't exist. The whole point of these techniques is to get the most benefit at the lowest cost possible. Results vary. Otherwise, what's the point, just apply massive amounts of super or downsampling... it will get the best graphics candy at the most prohibitive performance expense.
The issue is more complex... what is better? Excellent ambient occlusion and no anti-aliasing? Excelent anti-aliasing and no ambient occlusion? Or, if you can't get max, a little bit of both?
Having the option
of a lower demand TAA will allow you to combine it with DOF, AO, Reflections, etc... it's no use if you have awesome TAA, but you have to turn off other graphics enhancement features in order to play. The end result will be worse
graphics, not better.
ENB has always had options to use different qualities of filters, types of effects, etc... so that everyone can tweak the best possible results for the most machines out there. You can't make software for machines that have infinite performance, and you can't argue that you only care about users that have 3xTITAN Ultras on SLI, because no one will use your software if you do.