The cost of the domain name is $10 per domain per year or $1 per person per year little enough for one person to pay without thinking.
Most people keep an email address for a very very long time rarely switching unless a service ceases to exist.
Almost no domain owners are going to pony up $720 per year themselves and collection from other users will be an untenable hassle.
Basically every bullet point you listed is wrong. The remaining pool of users is likely 99% vanity users who would have used Gmail+custom domain forever at no real cost to Google over free Gmail none of which will migrate to Google workspace.
For Google the gain from this change will be identical to picking n random Gmail accounts and canceling them and keeping those users digital purchases as a giant fu.
Not ruinous but hardly profitable either.
Someone with an ounce of sense would have included the option to migrate all email accounts to regular Gmail accounts.