Looking at it a different way, there are two primary reasons ownership will spend:
1) A better team leads to more revenue, and thus greater profit even if the costs are higher.
2) Personal pride in winning a title. Owning a professional sports franchise is definitely an ego-boosting item, and rich people generally like to win.
In terms of #1, for the Celtics, the biggest impact comes from ticket sales and related purchases (happy fans buy more concessions and merchandise). Also an extended playoff run means more premium games to sell. The Celtics have their local media deal wrapped up until 2038, so while perhaps there are some clauses that increase or decrease the value based in ratings and number of playoff games, for the most part the variable is game-day revenue.
So say spending $10 million above the tax line will cost the team roughly $30 million in salary, luxury taxes, overhead, etc, will that net the team greater than $30 million in revenues from increased ticket revenue, merchandise, etc? After the disappointment of this season, I think the answer is probably yes. Fans are a little disillusioned, and will need some additional investment by ownership to energize their enthusiasm. Whether that comes via trades or free agency I couldn’t tell you, but given what I’ve seen in the season ticket thread, it seems as if the Celtics will need to put some additional investment into their product for a couple of years.
In terms of #2, I think that extra spend only comes if the team is close. Wyc doesn’t solely own the team, and the ownership group as a whole probably gets some say in the budget — when you have to get buy-in from multiple owners, the likelihood that ego causes extra spending is diluted. The Nets, notably, are owned by one person.