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.
The economic incentives behind mining on testnet and mainnet are fundamentally different, so I believe using testnet as a model for how things will unfold on mainnet is far from accurate. While the long-term sustainability of miner incentives isn't an immediate concern, it's still something worth considering. We don't want to end up with an ecosystem where a relatively low hashrate makes the network vulnerable to attacks by any moderately resourced asshole.
That said, the future of mining incentives is a complex topic on its own. Personally, I like to assume that the value of BTC will continue to rise indefinitely relative to the cost of energy and semiconductors. This has, more or less, been the trend over the past decade -- block rewards may drop in BTC terms, but their fiat value tends to increase. The result has been a steadily strengthening blockchain.