Modern arenas shouldn't need to be replaced. The most expensive part of an arena is the roof. Look at TD Garden or Capital One Arena, you just gut the inside.
Yeah, what's crazy is the Flyers/Spectacor just did that. To the tune of $400 million over the past 5 years. Essentially a whole new interior.
What this tells me is the Sixers scared the ever-living shit out of Comcast. Here's a potentially-long and not-comprehensive background on the situation.
Spectacor was Ed Snider's company. A few years before he died, it consisted of the Flyers, the Sixers, the Wells Fargo Center, the Comcast Sportsnet/NBC Sports Philly channel, a sprawling arena management company called Spectra, and a few other random holdings (lacrosse team, eSports team, other properties).
As Snider aged he began to divest to Comcast, which now fully owns Spectacor. He sold the Sixers. The TV channel was spun off into the greater NBC Sports group within Comcast. Snider's righthand man, Peter Luukko, was expected to take over leadership of the subsidiary, but he left abruptly during the Comcast takeover. Luukko was later asked by Bettman to oversee the Florida Panthers: he helped stabilize their arena situation and put the people in place who eventually built their Cup team.
Spectacor has since been run by Dave Scott and now Dan Hilferty, both outside corporate executives with no background in sports and arena management. The company has continued to spin off most of its non-Flyers/WFC holdings. Spectra, the biggest one, was sold to private equity...and the firm that bought it hired none other than Peter Luukko to run it!
So the Flyers/their parent company have been reduced to shells of their former selves. Meanwhile this whole time, the Sixers have been on an expensive lease in the Wells Fargo Center that benefitted them in no way. They also have an absolutely terrible TV deal with NBC Sports Philly, signed in the Spectacor days, which is paltry compared to what contemporary franchises get. NBA franchises, and the Sixers especially, have exploded in value over the last 15 years or so. It didn't make sense for them to be playing second-fiddle to a middling NHL team.
The Sixers had been getting the short end of the stick from the Flyers/Spectacor for decades. The Sixers, now owned by the Harris group, wanted what every sports team owner wants: That sweet, sweet bonus income that comes from owning the premier indoor events center in a massive US city, slurping up all the major concerts and political conventions and whatever else in the area. The Sixers leaned on city officials for the last few years to get approval for their plan.
Spectacor was contributing to the community groups opposing the plan, because it would have been a huge blow to the Flyers' value/Spectacor's income. But they have also been doing everything to try to convince the Sixers to stay. They just spent those hundreds of millions of dollars updating Wells Fargo Center, hoping to make it sexy again and avoid having to build an all-new arena anytime soon. They even recently offered the Sixers half-ownership of the Wells Fargo Center.
But Harris wants prime real estate, not a share of some aging arena. The Sixers ignored all of Spectacor's overtures and rolled on and got city approval for their arena plan last month...and then this comes out of the blue. And reporting is that it wasn't Hilferty who struck this deal--the CEO of Actual Comcast itself came down from his throne to make sure it got done, and apparently they only began talking in the past few weeks.
So, TL;DR...the Flyers/Spectacor used to be the big dogs in this relationship, but their local power has waned since Snider left the picture. The Sixers arena plan represented a grievous threat to their remaining value, and the Comcast overlords have decided it's better to have the hockey team accept a little brother role and reap SOME of the benefits than to fight, lose, and get none. But this is a BIG about-face for both companies.