I think its actually pretty reason to quibble the lack of transferability to a Senior Railcard if, as is quite probable with someone buying a Network Railcard at age 59 years 11 months and 2 weeks old, ite then has to be replaced with a Senior Railcard only 2 weeks later.
You can come up with your oh well they have still covered the £30 cost of buying it analysis but the fact of the matter is that someone in this situation will still be paying far more per day of network card validity (up to the point at which it becomes pointless due to the Senior Railcard's improved terms and conditions especially re nationwide validity) than in any previous year and that does not seem in any manner equitable or reasonable.
In your scenario above, you would still be entitled to a further 50 weeks of Network Railcard validity. If you had really renewed a railcard for just a two week period of non-overlapping use, you would surely have considered that matter very carefully before purchase, and decided that it was still to your benefit, in the full knowledge that you would have been eligible for a Senior Railcard just a few weeks later?
I do understand your point, but it’s not like a TV licence where you’re paying a year in advance for something that is actually metered and available monthly. A railcard is a one-off purchase with a year’s validity, and no obligation on you to travel each month of that year, for example. As has been observed, these cards can sometimes pay for themselves with just one journey. You can choose to work it out on a cost-per-day basis, but that isn’t how the card is sold, and nor (I expect) is that how most people think of it.
I do see that it would, theoretically, be nice for partial refunds to be available in these circumstances, but I suspect that for most people, given the very few opportunities in their life to transition between railcard eligibilities, the small cost of the railcard overall and the even smaller likely amount of that which would be refunded, you’d be running into one of those scenarios just like ticket refunds, where the admin cost of the refund could be more than you were entitled to in the first place.
My advice is genuinely to enjoy the fact that you can now take advantage of the senior card. The card you had before has presumably served you well and you have not lost out at all. The card you have now is even better, which is great! Look at it that way and there is no downside. Choose to be happy.