I say that extending the E branch all the way to Forest Hills is a bad idea for one major reason- Centre St is too narrow, and a lesser- the OL is too close. I base this on the three transit systems I am familiar with are the T, Toronto's TTC, and Minneapolis pre- light rail.
In Toronto, the streetcar routes added since the nadir of the system are on streets with four lanes of traffic and/or parallel parking in addition to the double tracked car lines- Spadina, Queen's Quay, and St Clair. Some streetcars run in streets as almost as narrow as Centre, but have two advantages JP lacks- parallel streets just a block away and wider side streets, both of which make driving and parking easier. Streetcars on Queen East, for example, run at the same speed as buses.
Consider how wide the surface Green Line streets are- Comm has a boulevard, sometimes two (lots of parking), and Beacon has angle parking on the median. An aside in favor of the F line to Dudley- although it narrows as it approaches the Pike, Washington St is 5 lanes plus parking- plenty of room for rails.
I looked at the 2010 Blue Book and the 39 bus had 14,000+ boardings on the sample weekday, as did the surface C branch, SL5 (aka749), and the 66 (my nemesis). That figure impressed me- if any bus was to be railtitued, on numbers alone the 39 are equal to the SL5, my first choice (F-Line to Dudley, if you like). However- we don't know how many 39 passengers boarded on the section parallel to the E branch, and how many boarded on the section that a renewed Arborway line would cover. If the 39 ended wherever the E trains terminate, passengers going from BB to LMA would have to walk up to Copley to take an E train and their boardings would not be included in the 14,000.
Why did I mention Minneapolis- because when I lived in St Paul in the early '90s the busiest bus was the University Ave-all-the-stops between the two downtowns. Those buses had more standing room (may have been longer, too), therefore more capacity. Better buses is a much cheaper alternative to a 1.9 mile rail line, (additional LRVs not included. Termsandconditionsapply) when the LRVs travel at the same speed as buses. Yes, they are removing two lanes on University for a LRV line, but again JP lacks St Paul's nice grid of streets.
Acceptance of people driving and the importance of street parking is not an endorsement of private cars over transit of any kind. Removing parking from 1.9 miles of main street is too much, especially when there are no alternatives,
"Romance of the rails" is a real psychological phenomenon that plagues transit planners. I read about it years ago in a dour, serious magazine (most likely The Atlantic), and it is a double edged sword. People who would never ride a bus would ride a train or streetcar. People who would never take a taxi, never mind carpooling, will vote for a rail bond issue they'll never use. They like the idea of rails more than tires, and don't think about the lack of advantages a particular rail project has. Building rail lines of any description is a hell of a lot more expensive than making buses better.
So, wait 10 years Arborway fans, get some numbers together, get JP businesses on board, waft the hint of a rail line under the big property owners, and see if you can't get the T and Beacon Hill on board. Maybe get the city to back a few parking garages along the route.