Bolivar replaced by Petro

The few tax-paying companies left in Venezuela will exchange their VAT or whatever liabilities payable at the end of each month directly for Petros, as the Bolivar worth will increase almost daily because of the hyperinflation and the base currency will be the Bolivar. Effectively being a net gain for the tax-payer. (Bolivar amount owed to the IRS stays the same, but the Petro will rise in Bolivars).
Normal people will tend to prefer to hold Petros instead of Bolivars. Only Crypto-Hating Venezuelans will stay in Bolivars and loose big time.

Swift exclusion of the normal banking system

Being the Petro a competitor to the US-privilege of the Petrodollar (Free worldwide credit to the US), at the very least an isolation can be expected by not allowing money transfers to the traditional banking system via Swift. Just like Iran was isolated a few years ago.

Traders heaven? Petro in Venezuelan Crypto-Exchanges

Venezuela's solution to the possible cut-off seems to be the establishment of 8 locally licensed exchanges. On these exchanges even though a USD-exchange might not be very viable due to the lack of possibilities to transfer the money outside of Venezuela if there is an exclusion of Swift.
But this is no inhibition of Petro/BTC, Petro/LTC or Petro/Steem trading pairs. There is no way that a BTC-transfer can be stopped.
Traders who are worried that BTC and the whole cryptomarket might go down, might consider the Petro as an alternative to the non-audited, subpoenad USDT Tether and park the money during downturns on those Venezuelan exchanges in Petros.
I assume that there will be no reputable exchanges outside of Venezuela offering Petro-exchanges due to sanctions.
Venezuela might establish the most permissive crypto-laws in order to allow the government there to survive.

Critic: No-one will believe in the Petro if it is not exchangeable for an actual barrel of oil

For decades we see the example of the Comex, where there are a 100 times more silver contracts than physical silver. No one seems to care that the settlement there is also always in USD, not actual silver. It would not even be conceivable to do it in actual silver by all stats.
In either case, according to the Petro Whitepaper Venezuela will promote the acceptance of the Petro also by PDVSA the state owned oil company, which if it is been done right would lead to something like a physical backing, even though the Petro is only a tax-credit for the price in Bolivars of a barrel of oil.

Mandatory intervention of the US to hold the experiment

The US has been fighting for their Petrodollar privilege for decades and has bases in 150 countries all around the world. If the Venezuelan experiment is a success this would endanger it massively, as other countries will try to get a piece of the free credit as well.
So I think we have to expect some kind of intervention from the US government.

• They can call up the Venezuelan government and tell them to keep the received money, but fail to hand over the promised Petros. An excuse like technical failure or whatever could be used. No one would be really surprised if that happened.
• They could replace militarily the government and install a pro-US government which stops the experiment. The ideal moment would be before the Petro is been issued. The second best moment soon after introduction.
  As the Petro could always be killed just by not accepting it as a tax credit anymore. I think even the announcement of a military intervention by the US would kill the Petro instantly.

Why if there is no intervention the Venezuelan Government should be motivated to hold its promises this time

Absent a military intervention the Venezuelan Government has already announced plans for the "Petro Oro" (Petro Gold). This ICO might be much bigger than the Petro... perhaps 50 billion? If they screw up with the Petro experiment, than they can scrap their plans and forget about the money from the next issuance.

Option to go back to a market economy?

If Venezuela has once again a viable currency in the Petro, and a new viable industry in the cryptomarkets, than the possibility exists to salvage the economy by changing course and re-privatizing many sectors again. That would be the smart thing to do... but I am not sure if the Venezuelan government is so smart... In the past one could not see any evidence of smart economic decisions...

Disclaimer: As always this is no advice for investment, only my opinion. By no way this is an endorsement of the Venezuelan government either... I just think that this time they really do something innovative which might help them to uphold their power a bit longer. But they are playing with the risk of a military intervention by doing so.