I don't even see any rationale to essentially remove all time resolution after a comment is a year old. It doesn't save any significant amount of space.
This is a direct comparison of the text length for new and old timestamps:
Over a year ago
Jul 13, 2014 at 15:53
The old ones are slightly longer, but they would be shorter if you omitted the time and only showed the date. Which I think would be absolutely an option for older comments, as the time of day isn't important then.
This timestamp is also not in a position in the new design where you'd need to save space anyway. So I don't think space-saving is a valid reason here.
Leaves just "it looks nicer" to me as possible justifications, and I just don't buy that. It would be anyway too much of a reduction in information, and it doesn't even look any nicer to me. Relative timestamps are nicer for very short durations like "5 minutes ago", but they don' have any value for long durations like this, especially if you also cap them at a year.
I also don't think these timestamps increase engagement, so I really have no idea why SE would do that, and so far I haven't seen any actual justification for this changespecific aspect of the new design.