My idea is thus to change this definition in bitcoin-core to be based on difficulty instead. (new clients may then follow)
Then miners will be forced to accept transactions using these new fee's as theres no other transactions to choose from.
I meant with "old block" I mean mining blocks based on tx's that are from Before a specific difficulty change, and "new block" with mining blocks based on tx's that are after a specific difficulty change.
Since the blocks Before a difficulty rise, will have higher fee's, it will be more favorable for miners to then choose to mine blocks that are based on old transactions, while if theres a difficulty drop, then it will be more favorable to mine "new" transactions (thus becoming a "new block")
I know that im using the "wrong terminology" here but its easier to explain that way.
Are you really sure? Since the txfee is basically pinned to the size of transaction, it means it won't follow the market when value of bitcoin rises (compared to fiat). That causes the market to saturate when the txfee becomes so high so its unfavorable to invest in bitcoin due to the "spread" caused by the txfee. When market is fully saturated, it causes the value to drop because nobody is buying and nobody is selling due to the txfee's.
"Value" of something (Money, goods etc) is just the definition of the relation between demand and supply. If demand is low and supply is high, the thing is worthless, if demand is high and supply is low, the value increases.
I know that block space are a very scarce resource, but still, if the requested txfee (that the client request per default) did follow the market price, I don't Think miners would have anything against it, because they would still get as much Money (in fiat) in transaction fee's.