I've seen that the frequent opposition to zoning changes protests the replacement of single-family or row houses with "high rises", as you did.
Why did you chose to use an extreme as contrast? That's not the only alternative.
What about if they are replaced with a low-rise, or a mid-rise? These offer higher density than row houses but without the same visual impact as a high rise. Some in Manhattan love their 5-story brownstones. I lived in a city with lots of 4- and 5- story buildings, and think that - if well designed - they have a very nice feel to them.
As to the "why", our economics system says that (in general) when there is demand either prices go up or supply expands. If the prices go up, then people who can't afford the new prices must move.
Our primary economics system isn't really designed with people's emotions in mind, so this can cause stress and turmoil, leading even to protests and violence. One fix is to enforce rent controls, which leads to a different set of issues.
If we don't want to force people to move, then we either must make SF less attractive ("protests and violence" can help!) in order to reduce demand, or build new housing to increase the supply.
Not all systems are designed for growth. As a famous example (at least for beer aficionados) the Trappists breweries only serve the purpose of supporting their monastic way of life. The prices aren't raised to match demand, so some people end up buying and reselling the beer for a much higher price. As a remedy, the monks limit how much a single person can buy in a period of time.
There are other economics models which may fit SF better. For example, in the co-op system, the residents own a share of the organization which owns the row houses, or the apartment. They can decide as a whole if they want to, for example, add a new floor to the building or gut and replace the building. Because it's a larger organization, with its own financials, it can afford to take on a larger task like that. The residents would want this because the larger number of people in the building would likely lead to a lower rent. While with single family ownership it's a lot harder to make this decision.
This is a very different model than SF's row houses. I'm curious about how the choice of economic model affects how a city might transform itself, but can't do more than hypothesize about it like I just did.