Optional, only in part. How many PC's are running with no 3rd party software at all? I bet you (and many people that complain about W10's telemetry) are running software now that is sending plenty of telemetry back, even if you don't know about it. It just so happens that Windows 10 is an easy target, not that it's particularly any worse than any other software in it's telemetry reporting.
If there's evidence of said telemetry actually being used for something illegal or underhanded then that's certainly a fair point to make, but where is this evidence? "OMG MS know how many pictures I've looked at!!!!!!111111" Yes, and? Them knowing how much the software is being used and how is likely extremely useful information for the development team.
Of course, if someone doesn't like what the OS is doing then they are free to install something else. Linux is available freely of course and is an acceptable desktop experience (I run it myself on a NAS here). Since 3rd party software is totally optional there's no issues at all with going that route