As simple as that sentence sounds, you have phrased this straight to the point and I haven't been thinking about it that way yet. But of course, this is quite simple economics and yet you nailed it in a very sophisticated way saying that we essentially need "sustained congestion".

Now a lot of people may shrug for a moment as temporary congestion is already giving most of us headaches, but with decreasing block subsidy, sustained congestion is what we want to keep the network secure.

There are some things I don't understand though.

These ordinals inscriptions, what are the fees those users are paying in order to get their ordinals inscribed? And who are these users because a quick search returned that the formula to inscribe is

Content size ÷ 4 * fee rate

In the link, the example used is a foto of a cat with size 20kb. Long story short, the price is

5,000*fee rate

For everyone here who wants to do the maths, blockchair.com gives an estimate for the recommended fee (I know there are more precise sources, but for the sake of simplicity), which at the time of this writing is

104 satoshi per byte, equating to

5,000*104 sats = 520,000 sats to inscribe a 20kb kitty into the BTC blockchain.

My final question: who the heck is flooding the network with ordinals when it is so freaking expensive to do that? What part am I missing or not understanding?

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As a conclusion, I think sustained congestion while keeping the network safe and functional (in the sense as it was originally intended) rather implies that the solution would be second layers. I say "rather" because I might overestimating the point that with increased block size, the number of ordinal inscription will increase forever. But someone has to pay for that, which is why I ask who these someones are that can afford expensive inscriptions?

A bit of a wall of text, but would appreciate if you guys here would enlighten me, correct me, provide your opinions.