The problem is that after , you still have low rewards, and you've already taken the past loan, so you can't do it again.
You can. It is possible to lock coins for a block 1000000, then 1000001, then 1000002, and so on. Also note that non-upgraded nodes won't pick it, so that kind of locktime means "this block or later", and this "or later" part is quite important, because if there are rules to always lock something, then it may be endlessly moved. Also, you can look at RSK, and see, that it is possible to distribute fees more smoothly.
What's an indirect disadvantage with rewarding in that manner?
It is difficult to code it properly, and to understand from the start, how it should look like. Also, it means there could be some problems related to floating point numbers, and to rounding them correctly. Because of course it is possible to use integers, and calculate everything in satoshis, but it is not obvious. When you see 0x5a827999, you don't recognize it instantly as sqrt(2), written as uint32.
Also, when you look at SHA-256 implementation, you won't see k-values that are computed on-the-fly every time. You see a huge table of constants. And miners would then need such tables for block rewards, because they don't want to take risk that their block will be invalid, because they took one more satoshi than they should.
But if I had taken a look in 2010-2011, in the golden years, when its source code was available, it had some serious talking, mailing lists were active, I might have had some, just in case it catches on.
Exactly, there were some signals that it might catch on. And now, there are no signals that something else than Proof of Work may catch on. It is the opposite: there are signs of weaknesses in Proof of Stake, which is why ETH has "de facto Proof of Burn". If people could unstake coins now, it would be a disaster. But now, that cure can be worse than the illness, because burning means that after reaching 51%, there will be no way to unstake those coins, and fix the situation, so they will be forced to use another hard-fork.
I wouldn't invest into any altcoin, because it is apparent that it wouldn't have the same impact.
And imagine that there could be some altcoin, that would have Bitcoin-compatible mining, so you could mine both at the same time. What then? It is hard to know upfront, what will be successful or not. Because NameCoin did it wrong, and the path for Merged Mining is not closed, as long as "Instead of fragmentation, networks share and augment each other's total CPU power" is still not true. As long as each network has its own difficulty, instead of following the strongest difficulty, and changing amount of coins, it is very hard to trace all attacks and react to them correctly. So, I think people deserve those gains, because it was like investing in altcoins today: there were some positive signs, but nobody knew the future. Not to mention about testnet3, that is better at allocating names than NameCoin, because test coins are considered worthless, so they can only be used for sharing public information, like names and some test results.