The purpose of, say, the Jobcentre card and 16-17 saver (and, to a lesser extent, the 16-25 and 26-30) is to help people who are often earning less/no money get to work/education (and, to some extent, leisure-especially for the 16-30 age range). Meanwhile, the Network Card was, as I understand it, started purely for commercial reasons (filling empty seats off-peak on London area commuter routes).
In my opinion, the optimal restrictions depend on the purpose of the card. In the case of cards designed to help people earning less afford travel, the less restrictions the better (conditional on remaining reasonably affordable for the railway). On the contrary, I don’t think the railway (and therefore the taxpayer) should be giving discounts for anything other than commercial reasons when they are intended to be used for leisure purposes/people who can afford the normal price to commute.
Therefore, in my opinion studies should be carried out for railcards like the Senior (since, as far as I know, over 60s are not poorer on average then other age groups-a means based discount could be introduced for those who need it, of all ages) to see if they are revenue positive or revenue negative. If revenue negative, I don’t see the problem with introducing more restrictions or even scrapping them altogether. On the other hand, it may be found that they are revenue positive (say, most journeys using them would not be made without the discount and the users still pay enough with the discount to make retaining their custom worthwhile); in this case, they should probably be kept as they are.
If revenue negative railcards were to be kept for other reasons (say, reducing road congestion), I see no reason why they should be age restricted. There is no reason over 60s with plenty of funds should have their leisure travel subsidised over people from other age groups. It would probably be better to introduce a national railcard available to everyone in this case.