What would have happened if Russia, Venezuela, and Cuba hadn’t had the leaders mentioned in the title?
There is one paper for each country that tries to address that really hard question. Spoiler: the countries would have been better off.
Was Stalin necessary for Russia’s economic development? (2013) by Anton Cheremukhin, Mikhail Golosov, Sergei Guriev, and Aleh Tsyvinski. (Full paper)
Therefore our answer to the ‘Was Stalin Necessary?’ question is a definite ‘no’. Even though we do not consider the human tragedy of famine, repression and terror, and focus on economic outcomes alone, and even when we make assumptions that are biased in Stalin’s favour, his economic policies underperform the counterfactual. We believe Stalin’s industrialisation should not be used as a success story in development economics, and should instead be studied as an example where brutal reallocation resulted in lower productivity and lower social welfare.
The economic consequences of Hugo Chavez: a synthetic control analysis (2016) by Kevin Grieg and Norman Maynard
We find that although average incomes rose somewhat during his time as president, they lagged far behind where they might have been if Chavez had not taken office. During his leadership, life expectancy grew while mortality and poverty rates fell. Yet we find no evidence that these gains are any different than they would have been with another national leader. If anything life expectancy rose slower than what the control predicted. On the positive side of the ledger, it may be the case that inequality fell faster than it would have without Chavez in office after 2003; but we see no evidence that his term in office as a whole reduced inequality more than it would have without him. While these results are not produced in a complete simultaneous model, they suggest that the observed reductions in inequality and poverty should not be attributed to Chavez’s leadership, and that Chavez’s legacy may have more to do with the harm caused to per capita incomes.
The Cuban experiment: measuring the role of the 1959 revolution on economic performance using synthetic control (2013) by Felipe Garcia Ribeiro, Guilherme Stein, and Thomas H. Kang
We claimed, as many scholars have already done, that institutional change caused by the Revolution was the main cause of the divergence from 1959 onwards. We also dismissed the claims that war, human capital, economic embargo, and economic policies pursued by the Cuban government were the main causes of Cuban economic failure. The latter hypothesis could be an explanation, but clearly the economic policies undertaken by Castro were erratic because all of them failed due to lack of market mechanisms. […]
Institutional change through the end of market mechanisms and the collectivization process is probably the main reason of the country’s economic failure. As Ward and Devereux (2011) assert, despite some social achievements, it does not seem that history will absolve the Cuban regime.
Yes, this is speculative, but it is also in line with what economic theory would predict. If you want to believe the opposite, you would need arguments to show why these studies, and economic theory in general, are wrong.