Where I live, cycling is much cheaper than public transport and astronomically cheaper than driving. If you have lost wages and still have to go places, that's important.

You're assuming that there's a choice between supporting an economic recovery and supporting cycling. I don't believe there is, certainly not when it comes to return on investment for €50 per person.

I see increased mobility, decreased congestion in towns and cities, less pollution and less income lost to transport costs, not to mention the health benefits from more exercise.

I don't see a "than" here. These vouchers are also a cash injection into French businesses and workers, and specifically ones who are both important from a policy standpoint and yet likely to get heavily beaten by the crisis, since everyone delays repairs in a crisis.