Just my personal opinion, but I've always had issues with this kind of mindset because:
The current shitshow on maintenance and service quality was because the T didn't do proper maintenance for so long, to the point where nobody knows how to do it and the institutional knowledge is totally lost.
The same also applies to network expansions and planning.
Keeping a stream of extension projects under design and/or construction ensures that people don't forget how to do them. We had that happen before GLX, which was why things were so out of whack there until John Dalton had to come along and bring in the knowledge to do transit projects. Let's not lose that until another generation is gone.
Totally agree.
To add to that, there is a timing aspect to the planning too. As others have pointed out, it will be hard to fund any big expansions without lots of new federal funding. We don't know when that will come, but there are signs it could be sooner rather than later. There's been a bit more federal investment in the past few years, there's been population shifts back into the cities the past couple of decades, and even cities in red states are building out streetcar networks. Public transit is having a bit of a moment, and a new era of federal money might be one election away. Maybe. We don't know. But whenever it does happen, we actually need to be prepared with inhouse expertise and plans at the ready. It will be a huge waste of an opportunity if there's a ton of federal money available and the MBTA is too broken to even use it effectively.
I've recommended the book "The Great American Transit Disaster," which in part looks at how cities used the money from the last era of federal funding for transit in the ~1970s. Boston was pretty on its game, it was ready for the money, and built RL to Alewife, RL to Braintree, OL to Oak Grove, plus bought up and started operating commuter rail all in the span of a decade. That's
massive. Other cities (the book points to Baltimore and Atlanta) had totally destroyed their public transit systems by that point. They still got federal money. But with no inhouse expertise, the planning fell mostly to politicians, who chose some high "prestige" but probably low value projects. They built expensive subway lines through low density when they really,
really needed a functioning bus network. Project costs skyrocketed because, again, no inhouse expertise. That era of federal funding ended and some of these cities didn't have much to show for it.
I don't think Boston transit could possible get to a point as bad as Atlanta in the 1960s. But still, whenever federal funding becomes available, it will be so much better to have plans ready to go, and the organization to pull it off.