Yes, that's fair. It will always be popular to save people a few quid, that's not in dispute. And if it encourages a "good habit" to support other policy initiatives, all the better. But being popular and saving people some money isn't the stated aim - that's encouraging people under 26 to use buses more (instead of cars, presumably).
I'm not sure it is genuinely going to increase that or, if it does, if that change will persist once they have to pay or their life moves on and buses are no longer meeting their needs. I just have the feeling that that age isn't an especially high car using demographic - especially in cities/ towns where public transport is currently a viable alternative to car use. So if buses weren't free, what would they do? Either pay to travel, or maybe walk or cycle if that was out of reach (which you wouldn't want to discourage)? Some may currently drive or carshare for convenience, but would the bus cost change that decision? You may have some discretionary travel shifting from car to bus, but for some things that still isn't especially convenient (grocery shopping, IKEA, going into the country where there's one bus every 3 hours, ferrying kids around with limited time windows).
We don't have a car for environmental reasons mostly, but realise we're in the fortunate position of being in a city with good public transport and even so are well used to all its inconveniences and annoyances (stress when running late to pick up from childcare and the bus breaks down, or stops because someone is smoking on the top deck, or just doesn't turn up). And can afford the odd taxi if needed. But even so, I turned down a job once with better pay than the one I took as it would be an hour and a half on the bus/tram out in a business park, though only 20 mins on the bypass in a car... Good transport links theoretically, but only from the centre of town, which would take 40mins at least to get to! Not impossible, but hardly attractive, and not something I was willing to do every day even if it had been free if I had any alternative! But equally, you don't want to run hundreds of buses across the country more than half empty...that's hardly environmentally beneficial.
I don't know what the answer is, and it's not going to please everyone, but I'm not sure the answer is always "well, doing something - anything - is better than nothing" if it doesn't actually meet your aim, even if it sounds like it should and wins you votes.