It sounds like you doubt that is Elon's plan. In my opinion, it's 100% certified and guaranteed. That's what he's doing, and only new insight of a monumental nature could change that.
Why would anyone use or want another online payment platform? What kind of question is that? Clearly, you are not thinking clearly. Have you never used PayPal? Are you unaware that credit cards pretty much make everything cost 2-3% more? I'm not speaking of interest expense if you don't pay off your balance each month, I mean everything online and in stores is marked up 2-3% to cover the fees from MC, Visa, et al. I cringe when it turns out PayPal is my best payment option (due to the high fees).
All of this makes the payment system a HUGE part of our economy, very inefficient, like a huge tax on all of us. Yes, credit cards and PayPal, et. al. make it easy to pay, but at a huge cost. Almost as bad, a credit card or a PayPal account cannot replace your need for multiple other financial institutions. What if there existed a low-cost on-line bank that integrated all the functions from borrowing money to buy a car or a house, making payments, earning interest, exchanging funds between individuals, buying groceries, paying rent, holding crypto, etc. etc. etc. that was reliable, trusted, intuitive and made a profit by skimming but a small fraction of what legacy credit card companies, mortgage companies, etc. take from every transaction?
In short, VISA and MC and a bunch of other multi-billion-dollar companies would be disrupted. And good riddance! The financial sector consumes ~10% of our GDP which economists say is like a big tax on us all (ideally, the financial sector should be closer to 2-3% of the overall economy because it's inherently unproductive from a primary perspective). Elon wants to spend about 2% of our GDP to make life interplanetary. If he can shrink the financial sector by even that little bit, then he can take the profits that would have gone to financial types and re-appropriate it to make life interplanetary (or whatever other good causes are on his plate), all without being the slightest burden on anyone (except for the displaced finance billionaires).
Lower transaction costs. Sounds like a win for humanity. Any questions?