The main reason why these coins exist is to not have value.
Then it failed. In both testnet3 and testnet4. Because both are actively traded.
If they did have value, they would simply be an altcoin.
Well, testnet3 is going in that direction. When Bitcoin Core will remove support for it in next releases, then it will be treated as every other altcoin. And a lot of people wouldn't even notice it, if exchanges and users simply won't upgrade their nodes, and stay with the old version, to use testnet.
I want to give them to developers or testers who need to test bitcoin features and functionalities.
I never saw any developer, who wanted to test something, and was forced to get test coins from another user. You can just disconnect from the network, wait 20 minutes, and then you can mine a single block on just a CPU, which will do anything you want. And then, you can reconnect with the network, and see, how your coins are reorged, so they are truly worthless.
So, testing things does not require getting any test coins from anyone. Just invalidating the last N blocks from 20 minutes, and mining your own block on top of that, with the current timestamp, will instantly give you some coins, without asking anyone. The only case, when someone needs test coins, which are accepted by the rest of the network, is trading. Everything else can be tested locally.
I was given a big amount (unecessarily big for testing)
If you truly believe, that test coins are worthless, then why do you think, that this amount is "big"? Everything multiplied by zero gives you zero.