Caste Slavery is basically "Authoritarian Slavery". And that Slavery is cut in favor of Worker Stratum bonuses + Stratified Society. Pretty sure living standarts still apply after Technological Ascension.
We'll see, as I said before, I'm happy with the change to Authoritarians and, additionally, I do hope that ascended synthetic empires can make full use of all the living standards and strata. We won't know until release, and history suggests that we should be cautious. Dev time is finite, after all.
It is always suboptimal to have two species contest for the same roles. There is a minor window of opportunity where robots are build when slaves have not grown to the proper population counts. But that can be hardly worth a permanent Civic like Mechanist.
Double check what I wrote, the organics and synthetics each have a role that they are better at, therefore they are not competing for it. Even without mechanist, getting robots up and running is not that hard. Remember that it is a minor priority as it boosts the materialist faction happiness (and likely will still boost faction approval in the new update). Additionally, with the job system, as you expand your resource districts the slaves and robots should each shift into their respective job specializations automatically as they are grown or built, respectively.
Exactly how much value civics like mechanist actually give you is always debatable. I would also make the case that it would be boring to only play the game with super-optimal civics like mining guilds, cutthroat politics, and slaver guilds. (and I hope the update takes them down a peg)
Non-Sentient/Enslaved Robots get no Energy penalty, because energy is the Robot equivalent of food. This was purely a balacne reason, that can change the moment the Robot Upkeep does.
That Slaves could even work unity Production was due to limitations of the Tile System, that the job System will no longer have. The slave can work the Farmer job, while the free pop can work the "Unity maker" job.
Indeed, that's why I said it can all change. I know I (and many others) have said how happy we are that slaves will almost certainly be easier to manage in the job system (especially for the AI).
But consider this, if robots can perform the same job for less upkeep and greater output, is that not an advantage? Will propaganda machines allow them to do unity jobs otherwise allocated to expensive specialists? We already know from teasers that domestic protocols allow them to be "entertainers", normally a specialist job (as can domestic servant organic slaves).
As to whether this is ideal or not, you have to consider the upkeep, housing, and output of free organics vs organic slaves vs enslaved machines. Additionally unemployment and it's relationship to your planet stability and crime may also be a factor (a real problem in the historical roman empire, for example, between free pops and slaves for labor).
The main point is, I completely disagree that it will always be optimal to either have organic slaves or robot slaves but not both unless the devs deliberately design it that way in the coming update, and there is no indication (yet) that will be the case. Indeed, the lack of a tile system should make it easier to optimize. The problem currently is that the benefits really are not worth the extra tile micro, unless, for an RP example, you wanted a ruling class that only ever touched buildings and tiles with research output.