There are usually over 2,000 transactions in each block. Charging $300 to confirm 1,000 transactions in a found block could nearly double pool revenue based on your example of $350k revenue per block, and charging $300 per transaction.
... assuming you can even find 1000 users willing to accelerate their transactions for $300... when most of the "stuck" transactions are likely for amounts less than that and/or the reason they're stuck is because the users don't want to pay large fees and/or are happy to wait.
Certainly, the number of users with "stuck" transactions due to mistake or misunderstanding or exchange/webservice sending with low fee seems to be in a minority... and I would wager is not 1000 users
per block.
Of course this is all guesswork... maybe there are 1000 users every 10 minutes who would be willing to spend $300 to get a transaction confirmed ASAP... but, personally, I would think they would have just used a decent fee to start with. ¯\_(ツ)_/¯