If the economy grows or becomes more productive with a fixed money supply - you have deflation. Fewer tokens buying more goods. There is no need for a growth in money supply for economic expansion, at least at the start only a growth in velocity.
But the faster/bigger that expansion the more deflation, eventually the deflationary pressure will get to big, and the expansion will stop because nobody will be able to locate a token to spend.
If the economy is stalling, you'd generally want to be able to do government spending, to immediately produce some velocity, but at the same time your revenues are declining.
This is why MMT is just out of step with the idea we want a smooth economy rather than boom and bust cycles. MMT does not give you spending power when things are contracting, you can reduce taxes but if people are to pessimistic to spend the money drifts to the side lines. When the economy is growing you can expand supply as needed but taxing just slows growth and leaves everyone asking 'why'. Of course you need to because otherwise you have unchecked monetary expansion which will be inflationary but you lack debt to point at and clear aim target.
Another way to look at it is with fiat dollars we are already there. But creating debt on a rainy day keeps a record of how far the pendulum was pushed on the rainy days, so you have an idea of how much growth to choke off with taxation on the good days, to maintain some long term inflationary expectations - that was how it was supposed to work. Then we broke that system when someone said "deficits don't matter". You have MMT anyway because does anyone really thing the government isn't going to default?
We have not seen a federal surplus in 25 years and even then it boils down to where you draw lines and what your accounting methods are. Nobody in either of our political parties has publishing anything remotely credible as far as a path to a balanced budget. To that end the debt is already irrelevant in the broadest context, if you want to know if MMT works you are already basically living it and have been since at least the start of the pandemic. You tell me how's it working out?