If you are 100% all in , no dry powder at any time, then yes; XIV/SVXY is going to take a lot longer to recover as vol contracts and S&Ps go back up, but lets consider a 50% cash portfolio at a local top:
Looking at recent years, I examined the August 2015 correction, assuming you have half in SPY, half in cash, or half in cash and half in short vol (XIV, SVXY or cash secured SVXY puts which would actually improve your breakevens when getting assigned on the way down)
Using rough estimates, buying close to the top and then averaging down close to the bottom:
(Obviously in real life there would be more legging in to find a bottom and legging out to lock in profit, "assume a spherical cow", however the difference between the two products would be the same)
~210 to ~186 , about 12% drop
Assuming a 100k account, we would have 50k allocated (238 SPY shares) and average down an additional 268 SPY shares.
Here we can see those two total purchases recovered over the next 2 years to about a 20% profit, completely breaking even by about March. This is a pretty traditional take on buy the dip, dollar cost averaging, etc, that you might see in a traditional portfolio.
Now lets compare,
~93 to ~48, about 49% drop
Assuming a 100k account, we would have 50k allocated (537 SVXY shares) and average down an additional 1041 SVXY shares.
This is about the same as the SPY recovery trajectory, but oh wait, the Y axis is different. Oh that's right its 4-5x return..
I visualize this as one of those cartoon slingshots. The correction is pulling down on the slingshot ready for takeoff. If you are averaging down anyway, then averaging down on something that drops 50% instead of 12% lets you both average down for your breakevens, but can almost double the amount of shares you add, further improving cost basis. How hard can we pull down on the slingshot?
While XIV/ SVXY weren't around in 2008, some backtests just based on the futures at the time implied at ~95% drop in short vol, compared to a ~50% drop in SPY. To put that in perspective from the example,
SPY from 210 to 105 would have 238 shares buying 476 more,
SVXY from 93 to 6.50 would have 537 shares buying 7692 more
As long as the system doesn't fully crack with all short vol funds closing, as long as there is a single thread left- then the "buy the dip" slingshot is still there.
I'm really avoiding doing charts just for chart's sake, but sometimes it is the absolute clearest way to convey something that would take paragraphs.