Many things are difficult, but that shouldn't be difficult at all: miners just include transactions that pay the most (if space is limited) or that pay minimum X bits, X being defined by the miner, (if the space is not limited). THAT'S IT!
You are assuming a functional fees market exists. This is not yet the case. Most miners are simply selling hashing power into pools. Basically, only a handful of pool owners can define X, and they don't want to constantly play with variables. They want to set up an optimized mining operation and let it run.
Bitcoin as a store of value is currently it's main use case (in my eyes) and a takeoff for micro transactions would kill it. I really don't know why there are so many who were thinking that bitcoin has a future for micro-txs although there is a fixed blockchain. (If at all, transaction replacements are the future in that regard)
This isn't about micro-tx. The fee structure change a year ago already made micro-tx non-viable, and push it off-chain. This is good.
The problem is that the block size limit is simply too small for Bitcoin to maintain reasonable growth, and it will hit a bottleneck just trying to process normal business.