… Have you ever seen an episode of breaking bad… 
I had a buddy of mine with a few GRAND worth of games on steam and he set his account to Verify on the mobile phone ONLY. Lost his entire collection because he couldn’t get back in despite thinking he had the password to it and the phone took a bath… valve was NEVER helpful, at all…
As for phone verification that doesn’t stop anyone from going to the dollar store and spending 15 dollars on a phone for each account they buy. and most of the time people can find an SMS spoofing service on the net these days.
Credit cards - I think you would lose about 3/5ths of the player base because a good portion of the people would not be qualified for Credit cards, plus most people need a decent credit score to sign up for a card this day in age compared to 2001. After Dodd-Frank act was passed the banks really couldn’t hand those things out like gift cards, and even after it was repealed they still can’t.
Plus I think it would be some form of discrimination and a violation of Civil rights. Even when it comes to Cell phones it already crosses the line. As I can see sometime in the near-distant future an attorney could essentially argue that to some extent for the lesser of a better term it breaches a person’s rights to use a service under the pretense of having to use a service to use a service… (idk)…