I think the founders would be hugely disappointed that so many members of the legislative branch have ceded so much power to the executive branch, which has filled that vacuum with corruption and assaults on the rule of law, even undermining the fundamental right to vote.

I think they would be disappointed with corruption and assaults on the rule of law. I don't know that they'd be hugely disappointed with the ceding of power to the executive branch. The whole point of having a separately elected President, rather than a Prime Minister or appointed executive, is to take power away from the Legislature. The Constitution was drafted at a point where there was a backlash to the incredibly strong concentration of power in the legislature in colonial/early State government, who mostly (though not uniformly) had the legislatures appoint their own Governors rather than elected them.

They had no experience with modern political parties, so they had no idea - and presumably no expectation - of how or whether their new structure would actually function in the context of political parties. And obviously they had a different idea of the structure of the country under the new government (think more like the EU than a single country). I don't think we know whether they would regard the modern development of a very strong Executive as a disappointment, or a clever kludge to adapt the system to the new realities of the U.S.