In a country with the third highest population density in the world and a solid advanced economy? (discounting city-states and low-population states)

There are worse places to make that bet.

If everyone makes the same bet, it can still lead to disaster (real estate must rise! Let's all speculate on real estate! --> eventual disaster).

Or let's put it this way: a rational bet is something like, this thing is worth $50 now, it will be worth at least $100 later. An irrational bet is one like "this thing will be worth more over and over again!" If everyone things like that, then the thing's price will keep rising until the bubble bursts.

That sounds like the reasoning used in the US before the big 2008 housing market crash.

The US is not #3 in population density. ;)

That doesn't matter, it's just a matter of time before it crashes imo.

The fundamental reason the US housing market crashed was that mortgages were written on properties that couldn't be sustained by income.

South Korea has two characteristics that directly impact that: scarcity of land (relative to population) and a solid, world-leading economy in many high margin industries.

It's not inconceivable property with such qualities could increase in value forever, and still be properly valued.

So saying "Just like 2008 US" is a pretty pithy distillation of a number of complicated factors.