To start of the discussion, I want to stress that history rarely repeats itself, but it often rhymes. BTC has, thus far, lost around 85% of its value from top to bottom after every bull-cycle thus far. Can the same be said for alts?
In the previous cycle, the total altcoin marketcap (which is the total marketcap minus ETH and BTC) lost around 92% of its total value, which translates to every altcoin losing around 92% of its value on average.
Since BTC follows a boom & bust cycle with the bust being around -85%, there's a reason to believe that alts will do a similar thing. In that case, alts have another huge 70% drop ahead of them. Naturally, this would be my absolute worst case scenario for alts which could only occur when there's a total meltdown of the stock markets (like 2008).
On the other hand, I doubt it will ever get this far. There's a lot of decent alts around and investors are not flocking to BTC as fast as they did in previous cycles, hence why the BTC dominance is still this low. Nevertheless, it never hurts to explore different potential scenario's.