The core reason SROs threaten larger spaces is that landlords can often extract more total rent from a single apartment by chopping it into pieces than by renting it as a whole.
Hypothetically: A landlord rents a 1,000 sq. ft. 3-bedroom apartment to a family for $4,500/month.
In order to extract more value, the landlord converts that same space into 4 separate SRO rooms with a shared kitchen. Even if they charge a "cheaper" rent of $1,500/room, the total rent roll becomes $6,000/month.
The Result: the SRO format is more profitable ($6,000 vs. $4,500). If landlords can legally choose between the two, they will naturally favor creating SROs over family-sized units.
Then there’s the potential for cannibalization of supply:
If SROs become the most profitable way to use residential space, the market may see a "cannibalization" of family housing.
Landlords of market-rate buildings may subdivide existing large apartments into SROs to capture the higher yield.
Seeing this, developers then planning new buildings will design them with fewer large family units and more micro-units/SROs to maximize revenue.
This reduces the supply of 2- and 3-bedroom apartments. If the supply of family units drops while the number of families needing them stays the same, the price for the remaining large units goes up.
This will then potentially lead to increased land value as real estate prices are determined by the potential revenue a property can generate.
If a plot of land can now legally host a high-yield SRO building (generating $100/sq. ft. in revenue), the value of that land rises.
A developer who wants to build a standard family apartment building (generating only $60/sq. ft.) can no longer afford to buy that land because they will be outbid by the SRO developer.
To compete, the family-building developer must raise their projected rents to justify the higher land price. This raises the "price floor" for everyone.