What people are suggesting generally is that you have what may be and certainly could be a very fine canoe, and that you might wish to consider giving it a quality restoration, even if it has seen its share of hard knocks. A 1921 AA-grade Ideal from 1921 is a nice canoe and one that, among Old Towns, is not very common. Properly restored, the beauty of that century-old mahogany against red cedar planking and white cedar ribs will be stunning.
Backside rib repairs are certainly warranted here. Yes, you may see a hairline crack if you know it's there and look for it, but if you do this right it will be essentially invisible to everyone but you. And done right your repairs should be structurally sound. And yes, it will look far better than the mismatch between most old wood vs. replaced ribs.
Do you have one of the good books on restoration? My personal favorite is this one:
Author: Jerry StelmokRollin ThurlowFrom the final decade of the 19th century until the mid 1950’s, the predominant small water craft for recreationists, summer camps, and wilderness travelers was the iconic canvas covered wooden canoe built by Old Town, B.N. Morris, E.M. White, Chestnut, J.H...
www.woodencanoe.org
and Mike Elliott has produced two restoration guides more recently, called "This Old Canoe" and "This Fancy Old Canoe."
These are great reads, loaded with practical information. I'd also suggest that if possible you find someone near you who can offer advice and maybe help. These forums are great and as you can see, people are very willing to share. But if you can get a hands-on guide beside you, all the better. I'm mentoring a first-time restorer now and it he's very fortunate to have such a ready source of experience (and specialty tools).
There was another thread here where a rough but restorable - and exceptionally rare - canoe was (in my opinion) fairly thoroughly destroyed. The owner patched it up and got it back on the water, but if that could be done then so could have been done a fine restoration, except for an unwillingness to take carefully considered advice. We often say here that it's your canoe and you can do what you want with it. Yours isn't the only AA-grade Ideal, and plenty of other canoes die of rot, fire and many other maladies. Even so, if this were mine I'd certainly want to do the best I could for it. If you do, you'll learn a lot, have fun, have much to be proud of, and will have saved a classic.