> I have yet to see an argument as to why a company isn't incentived to drop a health insurer if they're forcing employees _not_ to do a $50/month option and instead pick a $1k/month option.

That definitely does happen, but companies have a lot of levers to pull around how employees select health insurance. When I bought health insurance for a small firm (~60 people), I provided a set number of dollars and workers could use these for any plan they wished (an ICHRA plan).