Though, I generally agree with you, PDF/A is quite a good way of storing documents for long-term. But, that doesn't mean that PNG files along with text files, even with x:y coordinates next to the pieces of text, aren't a feasible alternative.
Djvu also supports a text layer just like PDF.
Note that 150PPI is barely better than FAX, so your documents will likely look 'faxed' if you ever have to output hardcopies for some reason.
In the US, permanently retained documents like court records are kept in PDF. It will be around.