The same was said over and over on the nascent days of the internet, about how it facilitated anonymity so easily and how it would be used by criminals.
> by stealing CPU cycles
Difficulty in bitcoin mining is raising all the time (because of the competition among miners). This is why nobody mines nowadays with CPUs or GPUs but with ASICs. The fact that some sites are trying to achieve some decent mining hashrate by pulling the amount of visitors of a website is a naive move that will soon be off-set by even more raising levels of difficulty. Soon this will not be profitable at all, even if your website was the 1st in Google's page rank.
> to the significant impact on climate change
I like that skeptics raise this point, because:
1) The cryptocurrency mining industry is the fastest one to chase energy-efficient solutions. As soon as a device consumes less electricity, it's adopted widely because of the huge incentives to do so (e.g. CPU->GPU->ASIC).
2) Compare the electricity spent by the entire banking industry with the electricity spent by the entire crypto-mining industry. Now guess what's more efficient.