Argument for a short period:
The "Elisha Gray Effect". If JW did not do it, someone else soon would have.
Argument for a long period:
From Savery to Newcomen was 14 years (1698-1712), from Newcomen to Watt was 62 years (1712-1774).
Argument for a short period:
John Smeaton improved the Newcomen engine in 1775 to a partial degree, probably independently, and Ivan Polzunov made an improved steam engine years before Watt, as did Humphrey Gainsborough.
Argument for a long period:
James Watt worked on the steam engine ~13 years, showing how difficult these major advances were, and was the one individual who really got the IR started.