So currently according to
https://twitter.com/CoreFeeHelper, a 11.7sat/byte transaction should get confirmed between 3days-1week. But if miners simply choose those transaction with the higher fee (and assuming than mempool size remains the same), then this transaction should never get confirmed as the higher fee transactions will always get priority over it. So maybe we're missing something here.
Transaction age shouldn't be a factor here to consider for adding into a block. It would minimize the number of people getting interested in mining Bitcoin, turning it into a less decentralized network. I'm not an expert when it comes to the technical aspects of Bitcoin so I can be wrong with my assumption.
When there's no greedy algorithm as per your point, miners will have an algorithm based on coin age, who will select the fees per byte? You can use 10 sats/byte or you can use 1 sats per byte. Of course, there will be fewer miners because the profitability will drop significantly because they will be forced to include all the transactions based the age. Fewer miners = Less decentralized.
If you fix a certain fee, say tx using less than 10 sats per byte, won't be included in a block, you are censoring the network. You can't fix a fee, you can't include all the tx regardless of the fee used in the tx. It's not going to work I think.