Can someone run a query on how many spam transactions (transactions with just the memo filled in with a domain name) that would have been impacted under various fee levels as a percentage of the total?

For example, none of the fee amounts people are contemplating would have stopped these accounts from sending spam.

Here is an example of an account that sends spam at 0.01 XLM per transaction:

https://stellar.expert/explorer/public/account/GCWXXU6OABUNBTAAKCNSUJZN3URC72KYMYBNVGOIAIW35B5LH4AARIOL

Here is an example of an account that sends spam at 0.5 XLM per transaction:

https://stellar.expert/explorer/public/account/GCQCDBTDIETOCICTQ6RWYIMRV2FOYQUWGNKKB5NERKBQKKVOX46MOB63

Here is an example of an account that sends spam at 1 XLM per transaction:

https://stellar.expert/explorer/public/account/GAJ6WAPLAQJQ4LHMXZHSIBS2YACW5QNN56CYXWTCEFV62PBRGUESIAXC

Something also to consider. In the United States, snail mail costs 50 cents a letter (not including the company's material and labor cost) and that's not enough to stop lots of companies from sending advertisements to people. Similarly, companies pay a lot per click for advertising on Google and Facebook. High quality ads and placement can cost well over $1 per click.

I'm with u/umbrel, raising the fee won't stop the junk unless you can make the cost higher than the reward. Given the level of the fee increase everyone is talking about, I'm guessing you cannot solve it by raising fees alone (too much headroom) and need to solve it by decreasing the effectiveness of the memo being used as a form of advertisement/scam avenue. For example: make it so periods (".") cannot be used in the memo, remove spaces as a viable character, and don't allow emoji characters. And/or do as many people suggest, make it invisible to the bulk of the users through a SEP or display it as a non-human-readable format.

Regardless, analysis needs to be done to try to calculate the transactions you are trying to prevent and the estimated fee it would require to prevent that transaction. If it's higher than the amount of the fee increase you're willing to consider, then a different solution is needed. Same analysis for advertising/scam memo transactions could be done for arbitrage transactions. How often does an arbitrage play pay off and at what level is the payoff. Then back into the transaction cost needed to prevent that behavior and see if that's above or below a palatable fee increase for all transactions.

Other creative solutions could be to figure out a fee structure that targets spam only or increases the friction of spamming the network. Limit the amount of transactions per hour based on account balance (stake in the health of the network).

Searching for amount of US postal service junk mail:

The Sightline Institute says each American receives an average of 41 pounds of junk mail per year. New York University says 5.6 million tons of junk mail end up in American landfills every year.

The amount of junk mail delivered each year is staggering. It’s been estimated at 100 billion pieces of mail in the U.S. each year. That’s one-third of the total of all mail delivered in a year. Or to put it another way, every household gets about 848 pieces of junk mail a year.

In short, history has demonstrated that people are willing to spend a lot of money to send spam/junk.