I use the SCP pretty much everyday and since this 10 vs. 12 feet debate has begun, I've started paying closer attention to use.
I'm not convinced 10 feet is going to prevent the extension from being extremely useful/capable of carrying relatively rapid bike/pedestrian traffic. I'd prefer the wider path, but I don't think that a 10 foot path with retaining walls on either side are going to be a big issue. Here's why:
1) I think the retaining walls and physical barriers (fences, etc.) will reduce some of the more leisure activity you see on the narrower sections of the path. The people lounging on benches on the side, the people walking dogs, people going to/from playgrounds, etc. won't be as present on the new segments. To start, even if it's 12 feet, it's still along a rail right of way with retaining wall(s) on the side(s). The current path passes by back yards, parks, playgrounds, and has benches for rest and a lot of space off the side of the path for leisure use. The new section, regardless of the width, won't have that. Pedestrians seeking those uses (parks/green space) will likely leave the path and walk on side streets rather than deal with the gauntlet of bikes/commuters.
2) Bi-directional traffic volume is not even. Yes, the current SCP serves both commuters and leisure users, but during peak times in the morning/evening, traffic flows much more heavily in one direction rather than the other. Things will change a bit when the extension opens and links new stations, but traffic will likely still be skewed heavily in one direction rather than two ways.
3) The placement of multiple stops along the route will decrease congestion along the length of the path. I walk and ride from Lowell St. to Davis. The volume of traffic increases significantly each time I cross a street. At Lowell, I can be nearly the only person on the path (along with a few dog walkers). By the time I cross Willow, it's pretty packed. Having new GL stations at at least three points along the route will induce usage of the path, but the number of stations will prevent congestion over long segments of the path.
4) I think the physical restrictions will force people to make better use of the space. Frankly, I notice pedestrians on the SCP tend to leave a lot of room between the edge of the path. Losing a foot on either side, combined with more bike traffic will force people to be more cognizant of the space. Combined with the fact that you'll likely see fewer dogs on leashes, groups of 3+ walking side-by-side, families with little kids on scooters or training wheels, etc. in the narrower sections, it'll probably be fine.
Again, I'm fully supportive of 12 feet. But I'm not convinced that 10 will be an issue, let alone the disaster some are making it out to be.