You don't seem to get that the money is spent when Congress allocates it, not when a public servant puts it on a credit card. You want accountability talk to your representative.
This will just make government less efficient as it goes through extra steps to request invoices and post payment, or perhaps even spending petty cash.
It will exclude government from buying certain products where this isn't possible.
Of course this is all quite irrelevant, since it's a publicity stunt for the people who are not interested in saving taxpayer money but hating and destroying.
If they need more they can get an increase, what's the issue
Again, wrong end of the pipeline. It's just more busywork to do. The staff is already responsible for what is spent on the card and to match it to approved spending and their budget.
Just building in more red tape and expense to a system.
Saying we shouldn't improve spending oversight because the real problem is at the budgeting level is like saying we shouldn't put security cameras in a store because the owner should have hired more trustworthy employees. Sure, hiring matters, but that doesn't mean we shouldn't also keep an eye on the cash register. If Congress allocates funds irresponsibly, that’s a separate issue—but ensuring that spending is properly tracked and justified helps prevent waste and misuse at the point where it actually happens.
Obtaining that increase implies further bureaucratic overhead. That's time (and thus money) that they didn't used to have to spend in order to get a job done.
You're also positing that the people in a position to grant the increase will do so in good faith. That's the opposite of Musk and Trump and their M.O.