Those are neither 'economic' disadvantages, especially from Bitcoin's perspective, nor are they meaningful or true.

Firstly, there is a strong incentive on the parts of the Blocksteam folks to design out counter-party risk, and as engineering problems go it is little or no more difficult than doing so in Satoshi's Bitcoin.  At least as long as Satoshi's Bitcoin itself can be relied upon in this respect.

Secondly, note that if one has a backing such as Bitcoin or gold, one does not need to rely on POW for this function.  One could use POW to a much lesser degree to shore up internal bookeeping to keep away the ankle-biters, but that's a secondary design issue and not terribly important.  The backing currency is the ONE system which actually NEEDS absolute security and Bitcoin has ONLY POW to achieve this.  Alts don't have this advantage since they tend to try to occupy the same place as Bitcoin.  Bitcoin can transfer it's security to other solutions with no net loss.  Just the opposite, really, because the more solutions relying on Bitcoin (and likely supporting native Bitcoin in some manner in most sidechains designs I would guess), the better.

The main thing that a user of a backing needs is for that backing to be solid and usable.  For gold this would mean some combination of physical security and transparency.  Gold was not discontinued as a backing currency because these things were impossibly to achieve, but rather because there was a desire by TPTB to get away from a non-inflationary base currency because they wanted an inflationary monetary system.  Achieving transparency and security for Gold would be, while possible, very much less efficient than with Bitcoin.

I think people lose sight of the value of POW in Bitcoin also.  Say you have a speeding semi truck that you wish to stop.  You build a wall of reinforced concrete which is 100' thick.  (Today's sha256.)  How much value is there to making the wall 200' thick?  Twice?  Nope.  There are other ways to attack the wall which will be explored, and a defender would be well advised to try to anticipate these and defend against them accordingly.

As nonsensical as it is to assign a simplistic linear value function in wall thickness as above, it is even more silly to reject any wall for any purpose simply because it isn't as strong as the massively over-spec'd wall for stopping the speeding semi truck.

As long as I have a reliable store of wealth in native Bitcoin, I care very little about the relatively less security of a sidechain (if it is even that much different.)  My uses of the two solutions will be entirely different.  True, in a sidechain failure I might get stung paying native Bitcoin transaction fees to get my money out upon a sidechain failure, but it's not an event that would be common but rather just a hypothetical issue which would give me confidence to use the solution and lessen the incentive for people to run bogus sidechains for the purposes of ripping people off (like some alts for instance.)

I do see it as rational to exclude some people from the highest echelons of monetary security (e.g., native Bitcoin.)  When I was younger I was sometimes lucky to have enough money in my bank account to pay my car insurance or buy a bag of weed.  Did I have some God-given right or entitlement to have my $25 balance secured in a Swiss vault on someone else's dime?  Absolutely not, and I never dreamed to ask for it.  My money, even if it represented half  of my life's savings was never at significant risk in part because if was only $25 on not worth the bother to steal.