At some point it feels like Monero has to go past Dash. The only thing I can really come up with is that the artificial controls for the Dash emission combined with the super high XMR inflation rate early on has really skewed with the market caps on both. I feel like it's pretty safe to assume that Monero jumps past Dash this year. After that I don't know.
If an anon currency came in that cleaned up and made Monero obsolete AND bitcoin tanked to the point of a market depression hitting and all alts outside of whoever is leading (bitcoin / ethereum) then I suppose Monero would drop further. But outside of that event it's really hard for me to see the downside being all that much compared to the upside.
Historically, there have been a lot of similarities in the ups and downs when comparing XMR and DASH against BTC. I really doubt XMR can move (in either direction) 4x compared to DASH in the near future, as that would be a crazy big de-coupling.
I think lots of investors are diversifying their privacy coin portfolio into both DASH and XMR, making that large of a decoupling unlikely.
However, with DASH moving development focus away from solely privacy features to other areas in the crypto-realm, that could make the market no longer see DASH as a privacy-centric only coin which could help or hurt.