SBDs are currently overvalued relative to the haircut price

They're actually not.

Fair enough, I inferred from the fact that conversions have slowed down and that it's hard to get reasonably-priced SBDs on the internal market that the price generally had gone up, but I didn't check.

If someone can get 25% from holding SBDs, but just 15 or 20% with daily posting for a voting service, the decision seems obvious.

The returns available from bot voting are more or less guaranteed by blockchain code, and the continuation of the status quo is partly guaranteed by bots and bot-voters maintaining lots of SP. Returns from SBD interest depend on witness choices which could potentially change arbitrarily with no predictable timing, and shifting assets from SP to SBDs would leave witness control up to those who maintain their SP. (Also, if you're imagining lots of SP getting powered down and sold to buy SBDs, are you factoring the price effects of that into your projections?).

if it doesn't work out the way I'd expect

You should establish some parameters for what you would consider enough of an experiment to conclude that this idea doesn't have the effects you hypothesize, because the same "we don't know, we haven't tried doing that exact thing" argument can be extended to but the rate wasn't high enough or you have to give it more time, or any other arbitrary thing.