There were other canals tried in New York to variable (generally lesser) success, for example the Chenango Canal from Binghamton to Utica. Most eventually closed due to competition from railroads.
The St Lawrence Seaway also took traffic away from the Erie. With modern ships, it is easier to send big ships the longer way. Barges are still useful for local traffic but the Erie was no longer the shortcut to Great Lakes. Railroads have advantage that can go more places.
> The St Lawrence Seaway also took traffic away from the Erie.
It also took traffic from the rail lines between Northern New York and points south, many of which are now disused. It also impacted rail in Canada, for instance the spur to the port of Brockville Ontario which serviced rail ferries to Morristown NY.
> With modern ships, it is easier to send big ships the longer way.
Interestingly, the Seaway was designed for the modern ships of the 1960s. The locks can't handle the new breed of giant container ships above the port of Montreal. Nowadays, the only ship traffic on the Seaway consists of "lakers" and bulk carriers. Small container ships disappeared long ago:
The early years of container shipping saw the operation of ships of under 1,000 TEU. Continued growth of international container-based trade resulted in the development of progressively larger ships, which eventually reached the maximum allowable vessel dimensions of the original Panama Canal, the early Panamax ships. Container shipping occurred along the St. Lawrence Seaway during the era of small container ships. However, these services eventually faltered, for multiple reasons.