The call for a boycott is a starting point. I don't think there are many Rovers fans who have not heard about it - so that is a start - get people thinking, get people talking and get the spotlight on Venky's in general and Pasha in particular.
It is an initial step and if most fans ignore it - as you suggest will be the case - then other options will be considered.
Things that will get more people on board are a run of bad results/relegation/bad transfer dealings. All those things will happen sooner or later under the current set up but if we wait until things are that bad, the call for a boycott will just appear to be reactionary. I think it is a good idea to 'get our retaliation in first' and it shouldn't stop after the Watford game whether there is 1 person there or 20,000 there.