we have to consider the issue of miners' income after years of insufficient block rewards
Even in testnets, like testnet3, miners can get a lot of coins from fees alone. People will use a lot of things, to spam the chain in many different ways. Changes to OP_RETURN alone wouldn't matter that much, when it comes to fees, because there are a lot of other ways to spam the chain, and most of the spam is done in much cheaper ways than OP_RETURN. Simply using future Segwit versions, and pushing everything as witness data, would allow to ignore many limits, and make 4 MB blocks.
Transaction fees currently account for less than 2% of miners' revenue, which is unsustainable.
The basic block reward in mainnet is currently set to 3.125 BTC. That's still a lot of coins. Meanwhile, in testnet3, you have 20 halvings, and 4768 satoshis per block, so everything else comes from fees (and miners can still get whole testnet coins).
So, I wouldn't worry about insufficient rewards for miners, because even in worthless testnets, miners can get a lot of coins from fees. And also, as long as 1 sat/vB minimal fee de-facto-standard rule is not lifted, miners are guaranteed to earn 0.01 BTC per block, as long as blocks are full.