Ask them for an example of a country where communism, socialism, fascism has lasted more than three or four generations. USSR lasted 70-80 years but there were two World Wars to divert attention from problems.
To be truthful, Cuba is nearing an exception to this, but they are geographically isolated and more easily controlled from outside influences. That said, it is no testament that so many of its captive citizens have gone to extraordinary measure to leave there, the IRON CURTAIN having been replaced by the Gulf of Mexico and surrounding.
Look further at South America now and see how governments like that are doing. Tell them to Google “toilet paper shortage” “food shortage” and so on as well as foreign loan debt and inflation.
It’s a no brainer.
BTW, why in the heck are you even talking to these people?
“Cuba is nearing an exception to this”
The real issue is: can they last without some external support or extraordinary assistance?
Cuba survives by sucking in external support via tourism revenues. Russia survived by conning the west into giving them wave after wave of food aid (North Korea is still doing this). Scandinavia persists because they have the good fortune of vast petro resources to draw on to prop up their economies.
Finding a socialist/communist country that can survive even a decade or two without a situation like that is going to be an exercise in futility.