It’s similar to how Citizenship works. A birth certificate (depending on country) is proof of Citizenship. A passport is proof of Citizenship. You don’t lose your Citizenship when your passport expires, or if your birth certificate is lost or destroyed. You may not have proof of Citizenship until you straighten it out, but the status (Citizenship) is separate from the document, just like the status of Permanent Resident is separate from the document (PR Card or PRTD).
I’m a Permanent Resident. At the moment, I don’t have a PR Card or PRTD (one card was stolen, and one PRTD was stolen, another expired). I’m still a Permanent Resident, and if I make it to the Canadian border, they have to let me in. I’ve done it with only a US driver license.
When I cross the border, they are free to examine me to see if they think I meet my residency obligation. I do, but if they think I don’t they can have a hearing. When I go to the hearing, I can present my evidence (records of travel), and any H&C concerns, and if I win I keep my status. If I lose, I lose my status.
The only one making me have a card is the airlines, and when I tried to use my US passport they wouldn’t let me come due to “non-essential travel”. I can still take my own airplane, or drive, or walk, or take a train. As long as they can verify who I am, they have to let me in. For people who aren’t visa waiver, the eTA is part of the difficulty of getting back, as without a PRTD you can’t get on the plane.