They cannot crash metals again like in 2008. The only reason they were able to crash metals then is that they crashed oil from $140 to $40 and oil is the main cost of production factor. The oil industry (and shale even more so) is way more fragile now than then and can't handle some massive deflationary event (nor pensions or anything else).
If such a crash occurred like 2008 again, there will either be an immediate economic and monetary reset to a new currency system/Bretton Woods, or even larger bailout than 2008 to reinflate the system, probably accompanied by something like basic income to try and sell it to the people
Overall it's way too problematic to let deflation happen when they can just try to cause mass inflation instead and not have to deal with these issues. If they let deflation happen with no bailout, banks won't exist anymore and metals will be the only monetary unit left because deflation wipes out the banks.
So what we have here is 3 variables:
Deflationary collapse with no bailout = buy all the metals you can find
Mass inflation to avoid deflationary collapse = buy all the metals you can find
Deflationary collapse with VERY BRIEF crash in prices before enormous bailouts = hold cash
More scenarios favor holding metals than not. Option #3 just means there would be a brief buying opportunity (days, weeks, a month?) before prices skyrocket higher than you paid anyway. If you wanted to cover all bases, you would hold 70% metals 30% physical cash (not in a bank) if you believe metals are already at cost of production.
All-in cost of production is supposedly around $1050-1100 on gold and $15-17 for silver. Some companies claim to get lower prices, but you have to factor in the average for the entire industry, not one guy bringing one ounce to market for less when the market needs a lot more than one ounce.
image source: valuestockguide.com