Can someone explain to me why the debt thing is an issue?

The US can inflate it away over time, with the main cost being the US standard of living staying flat or going down (which is annoying but very much tolerable given current American living standards). Moreover, paying for the debt by inflating it is a lot more politically palatable, as people don't usually think in inflation adjusted terms. Plus, you can take measures to fudge the official inflation numbers anyway.

Now compare this with climate issues, where no amount of financial engineering or political cost is going to fix environmental damage.

I'm not an expert but I can tell you why I think it's an issue.

The national debt has to be continually refinanced as parts of it become due. That works when investors are willing to give you money. But if investors think you are no longer a good investment they will continually ask for higher interest rates and then eventually they will stop giving you their money altogether. And if that happens then we're going to be in be trouble because we will have no means to pay what is due and we'll default.

The problem is the interest payment on the debt is starting to become a large fraction of the income of America and we are starting to look like an increasingly bad investment. And that shows in the rates we have to pay. I don't think we're in any immediate danger but spending goes up every year which means we need to borrow more and we move more toward that risky place.

And if history is to be any guide, when you hit the bad spot, everything falls apart fast in a very uncontrolled way.

So IMHO, we should ramp down spending slowly, in a controlled way to avoid having it ramped down for us in a rapid uncontrolled way. Either way, I suspect eventually spending is going to come down whether we like it or not.