Most people do not like surprises, especially surprises involving their work and/or money. Even if it may be a ‘good’ surprise, many simply do not like them.

How would a surprise day off work? “Surprise, you commuted into work today for no reason! Go home!”

Most workers desire consistency.

I can’t recall specific examples but I’ve definitely heard of folks that use patreon as a subscription thing and have the kofi for one-time donations. I don’t know their financials, obviously, but it seems to me that it works.