A fair point you have made.  However...

The issue here is not where the highest valuation can be, it is where it tends to gravitate towards.  There is little doubt that many who were mining SIGT when profits were significantly higher than ETH mining were doing so to turn the coin over into ETH.  And... when they did, that caused a wave of selling.  And, do not forget that when SIGT was 4000, it was both very brief and at a time when BTC was NOT in the mid 4000's USD.

My point is, SIGT hit quickly with strong daily mining profits and that invited the opportunity to SELL SIGT and roll over into more stable and mature coins.  That will cease to be the case in just a few hours.

This is just one part of the equation, but it is the part that, at least for the last 30K blocks, explains where SIGT can only seem to get about 20% above ETH mining returns before it finds a ceiling in value.

If someone is always there to sell just a little higher than what they want to roll into, SIGT cannot advance.  That opportunity will likely be gone in just a few hours and within a few days we will see what happens when these short term traders no longer can profit from their quick selling.