Blocks won't be instantly full when the limit is raised, so there's no point in going with not so big blocks. Unless you have another argument for doing it.
Yes, we know.
I want the spam limit to be lower, and not exponential.
This 20 MB + EXPONENTIAL growth would be an invitation for all kinds of services to spam the blockchain
No, it's not. Why would you think so? Transactions still need fees to get processed. People trying to spam will simply gave their transactions delayed or never confirmed, or it will cost them too much.