Retirement of and introduction of 6th gen fighter are two different things. Just by the timeline of 2035, it's easy to believe that China's 6th gen could come on board and J-20 may no longer be the spear-tip of China's air force but to retire J-20?? That means that by 2035, China will have an air-force of over a thousand fighters comprised entirely of 6th gens. I doubt that.
Well, he purposefully chose 2035, not 2040 or 2045. Maybe by 2035 not all bodies of J-20 are decommissioned, but the poem can only be explained as 2035 being the end-of-life, or cease-of-production, or some dramatic event. We can only explain things based on the info presented to us. J-20 has a major status change in year 2035 after defending the country (3rd paragraph).
So from his designer's point of view, it is end of life, because there is a better more effective choice. But from practical perspective, maybe it means start of phasing out/beginning of the end.
P.S. the poem was structured based on (whose writer just passed away a few days ago)