Three small counters:
On Seiya, his struggles and injury leading up to that benching are included in the overall team performance for the year. He was 37th in offensive value per FG for the year but 6th in August and September. I don't think that's a 'bad bet' to count on him for solid performance with the hope that he continued being the elite hitter he was the last 3rd of the year
Also baked into the overall performance was the abomination that was first base for most of last year, which Jed addressed. Yeah, you wouldn't expect Bellinger to necessarily continue to find enough 2 strike hits to get back to his 2023 performance, but going out and getting a bat first top prospect to play first instead of Mancini and Hosmer offsets a lot of that concern.
As addressed above, they weren't an average to slightly below average offense last year. They were above average, comfortably. Nico (102 to 93, career 99), Dansby (104 to 91, career 95), Morel (119 to 99, career 109), Bellinger (134 to 115, career 119), Happ (118 to 113, career 115), Suzuki (126 to 121, career 122), the oldest one of those being 30 years old, all taking offensive steps back and hitting below their career averages is just really hard to overcome.