This seems a reasonable deal to me. Of course, Id rater not pay UKP69, but I have had a long run of free upgrades - many of which that have brought enhancements as well as bug fixes - and there is a clear offer of more to come without further payment.
If Castle are to continue to produce RO5 upgrades (very desirable), to do these on a 'small but often' basis (good for Iyonix owners) and to earn revenue (essential for a commercial company), this "step in time" approach seems sound. The customer gets a concrete product immediately (can software be concrete?) and a promise of service for some period into the future; that promise is non-specific but can be judged against Castle's track record.
As a scheme, it's no worse than a Microsoft or Apple OS upgrade and better than some models we have seen.