When they built Epcot, they probably weren't even thinking about parks 3 or 4, and it was essentially a "given" that there would be a monorail between parks.
MK was intended to be the only park in the original "Walt" plans, as a reason for people to want to come to see (and live in) EPCOT, the city. Thus, they built the TTC so that when EPCOT was built, residents could simply take a monorail to the park. There were to be monorails and other forms of mass transit all over EPCOT (the city).
A new generation built parks 3 and 4, and costs made it prohibitive to continue building onto the system. Besides, since there was no city (per se), cars and buses made it such that adding onto the monorail would merely be a novelty, not a necessity.
So, I guess to answer your question, no, I do not believe they would have extended the monorail to Epcot if it had not been budgeted for.