But once the price starts going down for whatever reason, like a BTC rally, that ROI goes up in smoke and those same master node operators will run to the exits. Because they own so much of the currency, cause a snowball effect. Especially when bitcoin is rallying
You've described only one segment of a cyclical model - a scenario which has already occurred several times.
The bits you left out are the part of the cycle where the supply-demand curve balances out and re-asserts itself again. If you look at a 3-day chart of Dash since its launch you'll see that we've had about 3-4 full cycles of saturation/sell-off/re-establish demand so far and I've described this
more fully here in terms of a dynamic interaction between two segments of the coin supply - a "trading" component and a "reserve" component on which holders earn interest.
Sure, on top of that you have a risk asset effect against other currencies, but ROI on fixed income assets is usually measured in the native currency. They are two separate investment strategies - one accumulates Dash, the other uses Dash as a vehicle to accumulate bitcoin. Both are in effect and you can't use one to invalidate the other.
P.S. Your point about nodecount tanking the price when it flattens off doesn't square either. Trading is a 2 way street and also consists of 2 distinct markets: those investing in a reserve market for a fixed returns and those investing for a risk-return (buy low sell high). ALL of that trading has to come out of the blue segment of the coin supply in that diagram, which becomes ever smaller as the nodecount increases. The flattening off doesn't mean traders stop investing, it just means the supply's reached saturation. Trading will continue to occur, just on a much tinier portion of the coin supply.