Yes, the Celts won nearly 50 games this year (actually 50 including the playoffs) and have draft picks up the wazoo coming in, plus plenty of cap space.
Tons of ways to improve the team.
That said, we saw in the playoffs that simply removing a piece or two from the equation of this roster can make the rest fall apart. It's a deep team that relies on that depth to be as successful as it was this season.
This is a team with major strengths (defense, especially forcing turnovers, playing with pace, creating good shot opportunities) and major weaknesses (rebounding, free throws, finishing shot opportunities).
An important thing to remember is that improving this team will inevitably requiring swapping out some of the current pieces for better ones. In an ideal world those replacement pieces are perennial All-Stars and we immediately have a much better team.
In practice some of the swapping out and replacing might force the team to take a step back, even if it's intended as a win-now move (look at what happened with David Lee). We should not assume that the trajectory will always be linear and ever upward.
All of that is in some ways an argument to appreciate the team we have in front of us for what it is, since we just don't know when we will have a contender again, and it's always possible that necessary changes to get the team closer will mean experiments and gambles that go the wrong way and young players that need time to develop, all at the cost of beloved role players heading out the door in free agency and via trade.