So this is targeted to people that don't have cars? Just getting to/from work is 40 rides/month (average). Then add in getting around for shopping/dinner/etc. How much are these people spending each month instead of just getting a car?
I don't think anyone, including Lyft, would argue that Lyft is cheaper than owning a car for someone that commutes to work 5 days a week.
But that is actually a shrinking percentage of the white-collar workforce. Even huge companies now encourage office employees to work from home once or twice per week. So it could easily be down to 24-32 rides per month.
Also, the cost of car ownership is not just the car - you also have to have a space to park in. In cities, this can be quite expensive both in terms of renting or owning. Couples are more often trying to manage with having just one car (one person uses it daily to get to work, and the other person uses it for shopping/dinner/etc when needed but otherwise relies on a mix of Lyft and public transit).
So for someone living in an urban area, that can work from home at least once a week, this monthly Lyft plan might actually make sense.