I've always seen Suzy as a younger parallel to Susie, rather than her being Susie herself. Much like how Kris is an older parallel to Frisk/Chara, and Ralsei is a younger parallel to Asriel.
The main cast of deltarune being younger in Undertale has always pet peeved me as MK is really the only monster who appears to be older (Asriel doesn't count as he's been "dead" for years in Undertale). I theorize this is because he might be an orphaned boss monster in Undertale.
The only other argument I can think of supporting the Deltarune cast being younger is that Deltarune takes place in 202X, but that's still years before Undertale took place (As 201X is the year Chara fell down).
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