Which means any changes, bug fixes, etc. that is done to bitcoin core affecting the MainNet will affect all other networks including TestNet3.
I don't think so. Timewarp protection is tested only in testnet4, and is not moved into testnet3. And future soft-forks also won't be ported. Which means, that if you have for example testnet3 in v28, and it will be removed in v29, then a few years later, you may have Bitcoin Core v40, while using testnet3 will require going back to v28.
Of course, it is possible to get the latest master, modify the code, and use it on testnet3. But: even if you do that now, then you basically fork yourself, by splitting the chain into those, who enforce ASIC-mined block, during difficulty adjustment, and between the old nodes, who don't have this rule. Testnet4 will have test cases, which will be present only there, and then ported into mainnet. And you won't get for example anchor support in testnet3.
So, the end result of testnet3, would be just like a lot of different altcoins, which were created before Segwit, or before Taproot, and which simply didn't activate any new rules, even if they could. To stay compatible, you need some group of developers. If you have none, then you can just stick with old versions, which will get more and more outdated over time, and where more and more attacks will be possible.
I would disagree on the altcoin classification though.
Why? Each test network creates its own chain, and is altcoin "by definition". If you want to have a test network, which would not be an altcoin, then it would need to follow the mainnet. For example: if you would have a testnet client, which would allow you to put some regtest-difficulty blocks on mainnet, if nothing is produced for 20 minutes, and then your block would always be reorged, then you would have a testnet, consisting of just some stale blocks with low difficulty. And then, you can really say: "start from block 123,456, mine something on top of that, test things, and then move on, just by downloading next blocks, and destroy all of your test coins in that way".
So, do you want to touch non-altcoin testnet? Then for example sign some mainnet coins, test your things, and then destroy test coins, when they will move on-chain. This is the simplest non-altcoin testnet I can think of. If you have non-mainnet coins, then you have an altcoin, no matter if you like it or not. You have new coins, created from scratch. And an alternative history, an alternative coin ownership, and altered consensus rules. This is the definition of the altcoin.