Castro is a ruthless despotic killer. The entire economy is under his thumb, most of the prosperous larger scale enterprises are run by the Army. He purposely forbade minor economic activity (i.e. shops, trades) that would normally form 1/2 of the entire economic base. Some of this has loosened up.

You can't blame a blockade for not having mechanics available to fix cars when being a mechanic is defacto outlawed.

If Cuban were an open economy, even with crappy authoritarian leadership and and embargo, it would probably prosper.

Trading with other nations has some 'essential' aspects, but mostly it's not necessary, it's just advantageous. And there's no reason to have to trade with any one nation, generally.

Cuba might need very specific equipment from in order to advance some parts of the economy - for example, many factories need equipment only made in Germany as one example, the same thing applies to US sourcing. This would be acquired through intermediaries.

But by and large, trade would come in commodities, tourism etc. none of which is 'essential' between US and Cuba.

Not having the US to trade with is a drag, but anything that Cuba would actually need (i.e. specialized tech) would be purchased through intermediaries.

Definitely foreign direct investment and tourism would hamper things, but not by some kind of existential amount.

Castro Inc. is 100% the problem there, that people continue to defend them in some way is astonishing and shameful frankly.