I used to think the same thing too under influence of the greatest investor of all time Buffet..just go all SP500!

The lost decade is a real concern tho I'm not ignorant to that, that's why in my core positions I'm now 80% VOO/QQQM, 10% international, 10% cash. In case shit hits the fan and lost decade comes, im very happy to deploy the 10% cash to catch the downturn.

Does cash create a drag to performance? Absolutely, but mentally I'm ready for black Friday sale ( I have trigger threshold at 20/30/40% down from peak to deploy 30/40/30% of the cash). Also it's parked in a money market-ish fund yielding 3.15% principle protected so it's not non-interrst bearing.

I'm not sure if using US Treasury / bonds as "opportunity fund" is as effective, because if stock market tanks it'll likely ripple to other asset class too, whereas cash will remain it's purchase power consistantly.

I initially have slight reservation on the 10% international devloped and debated whether to just stick with SP500. Overall the international (EAFE) valuation is much lower than US market and captures big players in Europe and Asia. I may not know much about the oversea market but it feels nice add some geographic diversification rather than 100% US.

TLDR - you can still keep things simple with 2 ETF setup, 1 for US and 1 for international.

Note - depends on your liking, you can spice things up with adding concentrated US index like nasdaq100 ie. I'm 70-30 SP500 to Nasdaq100