All of these things are possibilities, but I don't really like "bubble," which I think more accurately describes the dotcom silliness of the late 1990's. Rather, we have an atypically low discount rate in the discounted cash flow pricing model. Interest rates have fallen faster than GDP has risen, so the effect on the discount rate used to value stocks has outpaced the effect of GDP on earnings (although cheap capital also enhances earnings). Some euphoria no doubt afflicts some players, but the discount rate is a real thing, not a figment of people's imaginations.

The question thus comes down to interest rates, which are mostly a function of anticipated inflation. That makes #3 the key to the whole question. Will there be a sharp increase in inflation? Will too much money chase too few goods? We have lots of money available to chase goods, but is it widely enough distributed to do the chasing? And what do we know about the negative output gap? What is the marginal cost of the goods that the money will chase, when it starts chasing it? SOME things are in short supply, but it's not clear that a market basket of things will remain in short enough supply to materially affect long-term interest rates, which drive stock prices.

On the one hand, I think we are overestimating the inflationary effect of coming demand. On the other, the effect of interest rate increases is much more dramatic at low rates than high rates. An increase in the ten-year bond from 1.5% to 3% will hit stocks much harder than an increase from 5% to 6.5%. That's just how math works.

But I don't believe there is anything "artificial" about the current level of rates; if there were, the markets would have already priced in the damage such rates would do. Right now, there is no reason for a stable government to pay people handsomely not to spend, which is what the interest on the debt of a monetary sovereign does. That won't happen until demand exceeds our ability to ramp up supply. Then, to control inflation, the government will have to pay people not to spend, or take their money away by taxing them, both bad for the markets.

The MMTers are right: inflation is the name of the game. Everything else, as the wise man once said, is commentary.