The Thomas-Dumars-Laimbeer Pistons were terrific from 1983-84 (49-33) to 1987-88 (54-28, lost in the NBA finals). It took them six years to finally break through as a group and win their first NBA title.
The Jordan Bulls were terrific from 1987-88 (50-32) to 1989-90 (55-27) and actually made the playoffs in 84-85, 85-86, and 86-87 (albeit with sub-.500 records). It took them seven years from the start of their playoff run to break through as a group and win their first NBA title.
Sometimes it takes a team a while of banging their head against the ceiling before breaking through. Those Bulls and Pistons teams suffered numerous heartbreaks in the playoffs and maybe to their fans it seemed like they'd never get over the hump. This doesn't mean Boston WILL - lots of really good teams never did (the Barkley Suns, the Ewing Knicks, the Moncrief Bucks, etc.) - but this team, as Strike4 said, is YOUNG. Their two best players are 22 and 23. They've got only three guys who's 30 or older (Wanamaker, Hayward, Kemba, the latter two recently turned 30). Though it seems like he's been around forever, Smart is only 26.
There's TONS of room for growth for this team still. Even if this playoff run comes to a close sooner than any of us would like.
But I don't think it's over yet. I think they get that first win and then confidence returns. Miami is very tough and experienced. But I think Boston can still win this series. Obviously game 3 is a must-win at this point.
Everything you state is correct. OTOH, we can never really predict how long windows of contention will remain open. The team is young, but it remains a question as to whether the Celtics can take advantage of that youth and truly contend for multiple years, or remain Conference Finals fodder. Plenty of young promising teams have ended up in the latter category, including the vaunted Bucks.
The window with this team is not guaranteed to be all that long, either. The Celtics can, and should, be able to lock up Tatum until 2026 with an extension this summer. Brown will be around until 2024. But Marcus is a free agent in 2022; Hayward will likely be one in 2021, if not sooner; and while Kemba is locked up until 2023 or 2024, he's unlikely to be KEMBA! during this period.
Meanwhile, competing teams do change. The Nets will have both Kyrie and KD next season; I do think Kyrie will decline rapidly, but probably not next season. The Bucks may yet get over the hump. The Heat were operating under the radar this season, but they are not going away, and Butler has been the best player on the floor this series, and Bam arguably second best. Worse, Miami is one of the few Eastern Conference teams that is a destination for free agents, and they do have some flexibility to free up cap space if I am reading their cap situation correctly. And the Sixers shouldn't be counted out.
It's not all doom and gloom, obviously. The Celtics do have room for organic growth, and if Haywards opts out and resigns, they may be able to free up enough space to use the full MLE this offseason (don't quote me on that). There really are no Super Teams in the East. A lot of the league's best players are in the West, some due to free agency (LeBron, Kawhi), trades (AD, PG, Harden/Westbrook), others due to skilled drafting (Doncic, Lillard, Jokic, Steph). There remain uncertainties about how KD will recover from a serious injury at 32.
Still hoping for a rebound in Game 3; win that one, and anything is possible.