It's true. But let's see how long testnet3 remains alive when Bitcoin Core and other software dropped support for testnet3. AFAIK testnet1 and testnet2 no longer alive today.
Similarly nowadays if you use a non custodial wallet, by its wallet default setting, you will be given Segwit addresses. Time for Nested Segwit addresses or Legacy addresses is the past few years and in future, more non custodial wallets and even more centralized exchanges will stop supporting creation of new legacy and nested Segwit addresses.
Testnet blokchains work only if there are miners on that Bitcoin testnet generation. If miners stop mining on a testnet chain, it will be a dead chain even difficulty on testnet can be reset to 1.