Steel beams have more deflection than microlams, about 99% sure. Also the deeper the microlam width, the longer the span it will carry. I do think you could carry the load with 2 posts, and probably 3-5 lam sisters per opening, which is more like 3 - 10' spans. These posts could easily be brought into your plan. However, if this were my structure, I'd probably try to design it so there were some wall sections. I just am not personally comfortable with the whole 30 feet of a 2 story colonial on 4 bearing points (2 os walls and 2 posts), here's why.
The amount of weight bearing on those 4 points is considerable, it includes the second story platform and walls, portions of the ceiling on the first floor, the original roof, and the new roof (probably a gable with the peak at 14 1/2", which implies a significant roof load potential, depending upon your locale. Technically, the bearing points for the roof would/should be on the outside walls, but I would still be inclined to give the roof some consideration.
Given these concerns, although microlams can do great things, I'd be inclined to mix it up a bit between posts and full walls, and I simply don't care what anyone else says, there is a lot of weight here.
Another consideration ought to be the footers you 2 story sits on. Some homes built in the 20s had really lousy footings (like mine) with about a 2" depth. This footing as far as I'm concerned are not sufficient to accomplish the goal of 4 bearing points for a structure this size.
Take some time with your plan, and make some short walls, you'll like your home better in the end, and you'll never be concerned about the house shifting or settling.
Additionally, if you are in an earthquake prone area, the whole plan would change.
No matter what you do, make sure you get the opinion of a structural engineer, and not the opinions of some guys on the FIXITFOREM