when we have a very useful decentralized currency that is working 24/7 for the whole world, then the amount of energy that it takes to secure this network is negligible no matter how much it is.
I'm beholding a completely unethical attitude or you've probably been poisoned by the over-enthusiastic thought that Bitcoin is that needed. The amount of energy it takes is
not negligible. It may not consume much right now, but if you notice it it increases analogously with the price. If we ever reach a time in the future when 1 BTC = $1M, mining will be much worse than before.
at the same time when we have thousands of useless altcoins (that is the cryptocurrencies) that have no purpose and are only pumped and dumped then even a single electron being spent for them is wasteful.
That is just your way of seeing this. There are people who find Bitcoin completely useless and thus, their opinion about mining will be justifiably against your statements. There are also people who believe in altcoins and they surely not find them useless.