Guys, the Erie Canal was built with funding from...the State of New York and, mostly, citizens of that state. Whether or not they do so has absolutely nothing to do with the presence of an American controlled St Lawrence if that river is under the control of a different state.
You still don't understand.
The canal made New York City the US Midwest's sole Atlantic trade port. The Midwest's primary export, grain, could reach the Midwest's primary export market, Europe, without first passing through Britain's hands. That made NYC an international trade center and the largest US city when it had earlier just been one of several coastal ports. Cities along the canal boomed as well. Buffalo, IIRC, grew from something like 500 people to 20,000 in only twenty years thanks to canal.
As you note, the gains New York state and NYC made from the canal spurred imitators. Pennsylvania spent millions trying to plug into the same trade via canals and later railroads, but Pennsylvania never managed to recreate the same boom New York state and NYC received because because the "all-US" trade route was already fixed thanks to the canal and New York-based companies continued to upgrade the route with canals and railroads of their own.
Quite simple, Pennsylvania was in an infrastructure race with New York and New York's early lead proved insurmountable.
If New Yorkers feel they want to be able to compete with the St Lawrence, they're going to try regardless of whether or not the St Lawrence is British or American.
If New Yorkers feel they want to compete with an "all-US" trade route using the St. Lawrence to move Midwest grain to Europe, they'll build a canal. However, just as with Pennsylvania's projects in the OTL, the New York project(s) will be trying to siphon trade and the money that comes with it from a preexisting trade network.
Not only that, a trade route which uses the St. Lawrence will be less "inter-modal" and therefore less costly. In the OTL, grain reached Buffalo by ship, was broken into loads canal boats then carried to Albany, was transferred to river transports to reach NYC, and was loaded on oceanic transports there. In an ATL in which the St. Lawrence is controlled by the US, grain will reach Niagara by ship, be transported by canal or wagon to Lake Ontario, and then loaded onto oceanic shipping.
Fewer changes of hands, fewer changes of mode, less break bulk, less infrastructure, less wastage, less cost, less time, more profit.
Could there be a canal linking Lake Erie/Ontario and the Hudson in an ATL in which the US controls the St. Lawrence? Most certainly.
Will such a canal provide the same benefits and economic boom the OTL Erie canal did? Most certainly not.