No, because if it's true that nobody is willing to buy your home, your home isn't worth the $300,000 you're claiming it is.
There's no single metric that works for all situations, but net worth is the best way to compare financial health between people and across a population in a meaningful way.
Similarly, for your investments, if the stock market takes a huge hit, your net worth will instantly become a fraction of what it currently is.
Cash will still be cash. (But also maybe worth less!)
There is no asset (including cash) that is guaranteed to preserve its value across arbitrary amounts of time under all circumstances. Even TIPS could theoretically be worthless if the government collapses.
That doesn't mean that net worth isn't still an excellent normalized comparison between arbitrary people or parts of a population today, which is exactly what we're talking about.
For example in a market where nobody is willing (or able) to buy your home, your net worth can be ... well, worthless. In a financial crisis it's even worse.
Your "on-demand" cash is what's in your checking or savings account. There's no guarantee you'll be able to tap into your home, car, 401K, etc. and even if you could, in the long term it may be more damaging to you. For instance in a city like Seattle if you sell your home, you could be looking at paying double or triple your previous mortgage in rent, so you're ultimately putting yourself in a worse position.