Sure there's a potential downside of complacency, but there's also several potential upsides, a big one is that there's still doubters (easy playoff run due to injuries to other teams) and a lot of viable opponents they've yet to face.
When Golden State was dominating, they seemingly beat Houston and Cleveland every year. They faced and took down all the superstars and potential threats: AD, Lillard, Harden, Durant/Westbrook, LeBron (everyone except a healthy Spurs with Kawhi). They beat every MVP, MVP candidate, and All-NBA 1st teamer (except Kawhi), while also having their own MVP and later added another MVP candidate on the team. Easy to see how they could get complacent.
But the Celtics don't have an MVP, won't have the best player on the court, the Sixers and Bucks have retooled since the Celtics last beat them, the Celtics have never faced the Nuggets, Wolves, OKC, or new look Knicks in a 7 game series. They haven't beaten a healthy Jokic/Murray Nuggets in 3 years (0-2 last year, and Murray missing 3 of 4 games in '22 and '23).
And repeating is hard, 6 years without a repeat champion.
Hopefully this all keeps them motivated next year.
(edit - see jpotter posted a similar thought as I was typing mine up)