If you do't intend to keep the title, then don't take it.
If you don't want someone else to keep the title, don't give it to them.
(Note that for positions subject to regular elections, you can change that to "...keep the title until the next regular election".)
The placeholder rule is intended to prevent situation where Alice loses the position (prison, wound, etc.), and Bob is given the title until Kepler gets back in two or three days. If Bob keeps the title until the regular election two weeks later, declines to run, and Alice wins in an unopposed landslide, that is NOT placeholding.
If Sir Reformer doesn't intend to keep the rulership under the new regime, he shouldn't be running for it. There is absolutely no purpose served by running for ruler in order to be the one to click a few links and step down. Elect the guy you want to lead the new regime, and have him do it. Of course, if you're switching to a regularly elected ruler position, you could elect anyone to click the links, then let him simply not run in the next election.