My analysis shows the opposite, actually. SBI units stack for the members, and votes are delivered when minimum thresholds are exceeded.
In effect SBI is a parallel curation mechanism, where you pay liquid STEEM to curate an account instead of paying an upvote to curate content. Then SBI curates the future content of that account. And incidentally, SBI has always been 50/50. The curator (sponsor) and curated creator (sponsee) each get 1 unit from the enrollment.
SBI will pass the extra curation earned by the voting accounts directly to the members, so that 'after curation' rshares delivered will remain constant. The impact of downvote pools and Convergent Linear is harder to predict, but I expect them to make promotion services more necessary instead of less. Every post will need enough promotion to get onto the 'linear' portion of the curve, and SBI is still the best 'set and forget' option available to make that happen.

My full analysis is in these two posts:
https://busy.org/@steembasicincome/sbi-in-an-hf21-world-part-1-50-50-rewards
https://busy.org/@steembasicincome/sbi-in-an-hf21-world-part-2-convergent-linear-and-downvote-pools