recent plague of foreign scam callers

If you are using Telus or Koodo for your phone provider, turn on Call Control. It blocks every one of those robocall based scams. If you have another provider, you are likely out of luck.

• https://www.telus.com/en/support/article/call-control-for-mobile-devices-explained
• https://www.koodomobile.com/en/help/call-control

As for your other questions:

• Caller ID technology came out many decades ago and is a global standard, meaning no one telecom can change the standard. The ability to display a number other than the one you are calling from has been part of it since the beginning. It's used heavily by businesses, government agencies, etc.
• Phone companies also aren't going to just block calls from random regions of the planet because not every call coming from those areas is a scam call. Unless the CRTC changes regulations, providers aren't going to be blocking calls like that.
• The extreme majority of the scam calls are using VoIP for phone service, some using services provided by some less scrupulous small Canadian service providers. A common one that pops up on the forums are VoIP numbers owned by Iristel. Look up their Google reviews. CRTC does nothing to stop them. When the scammers use a VoIP provider, they get a local gateway/phone number so it looks like the calls are not coming from overseas. There are other common US based VoIP providers that get used like TextNow, which also can provide Canadian phone numbers to customers/scammers. There are also a few other very small Canadian VoIP providers that seem to provide numbers for scam calls but that are harder to identify.