Turnover in SF is actually quite robust. The vast majority of tenants don't stay in the same apartment for decades. This is especially true during boom periods. IOW, ebbs & flows in turnover actually tend to benefit landlords. But a small number do. The average turnover rate is ~20%, which means the average occupancy is ~5 years. For large buildings what matters is the average. The fewer the number of units (either in a building, or in your portfolio), the riskier things become.
The riskiest of all is a single-family home, whether you rent out an in-law or the whole house. In the face of rent control you're not only risking being stuck with the same person indefinitely, paying an increasingly below market rent, but you're locking up for an indefinite period what is probably your largest and most important asset. It's a potential financial nightmare, and one with realistic odds (e.g. maybe 1 in 10 instead 1 in 10000) of befalling you.
As the building grows larger, the more the risk is spread and you can rely on the average occupancy term. Moreover, the larger the building, the less likely one would want to tear it down to build something larger. IOW, the larger the building, the more closely your expected future investment returns approach non-rent control levels.
Rent control policies should be accompanied by policies that promote densification. All forms of rent control will tend to reduce supply to some extent (at least, that should be the a priori expectation), but rent control also provides important social benefits. IMO, those lines cross, but I would never expect the benefits to be greater than the costs in the context of single-family homes.
The 1994 law encompassed multi-unit buildings of 4 or less, which were previously excluded from rent control. An elderly neighbor of mine owns a classic 1960s-era 3-story, 2-unit apartment building. He lives in one unit and keeps the other vacant so that his children won't have any trouble refurbishing or selling the asset when he dies. This is a nearly identical scenario to someone living in a single-family home with an ADU. There are several such homes on my block, though I don't know the owners so don't know how many are kept unnecessarily vacant.
If you want a £500,000 studio flat 30 minutes away from central london by tube there is a massive amount of supply. If however, you can afford said property, you probably dont want to jam your wife and 2 kids into it.
https://web.stanford.edu/~diamondr/DMQ.pdf
In terms of London, there is also the problem of building restrictions. I don't think we're going to see tall residential towers popping up in say, Mayfair, anytime soon. I understand that this kind of change would completely transform the character of London. But the price of not wanting change is sky high rents.