I would argue it was a significant percentage that made the jump. The 2600 sold roughly 30M units in it's lifetime; other contemporary consoles account for another ~7M units.
The Commodore 64/128 alone sold ~22M units. You also had the Atari 8-bits, the CoCo line, the Apple II, ZX Spectrum, Ti 99-4/a, etc. that account for many million more. In very broad strokes, you have roughly (+/- 10%) the same number of 8-bit home computers and pre-NES consoles sold.
Obviously there are a lot of other factors at play that make a 1:1 comparison impossible - 8-bit computers used for tasks other than gaming, both consoles and computers purchased as a response to the current fad but never really used, 2600s and 8-bit computers purchased *after* the introduction of the NES, etc. And simply the fact that we lack hard sales numbers in many cases.
Anyway - did everyone who owned a pre-crash console move to a home computer? No. But I think there is enough evidence to indicate that more than a "small percentage" made the jump from consoles to home computers as their primary gaming devices prior to the NES.