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Question for you: if people want to pay a miner to validate a transaction ... of 90kb today, how would they do it?
The way you pay miners a transaction fee is by having the value of outputs be lower than the value of the inputs you are spending.
Would other miners reject that block?
Unlikely, unless they mined a competing block at the exact same time.
If miners all adopt the default OP_RETURN limit today, why?
Miners adopted a higher OP_RETURN limit because they wanted to satisfy their paying customers.
Wouldn't miners be influenced by market signals?
Yes, the miners are highly influenced by their customers who are paying them to store their data on your Knots node.