You aren't wrong. Hundreds of thal minors who are members of this group agree with you. For some reason, doctors want to lump all minors together, even though their Hb levels vary greatly and so do reported symptoms. Unfortunately, your doctor may only see a few cases of thal minor in his life, so he won't have enough experience to realize that what the text books say about thal minor, is not true for all minors. You are right about the altitude, too. After about 3 weeks at high altitude, your Hb level will rise, and runners and bicyclists both train at high altitudes for this reason. High oxygen level=better performance. So why don't doctors accept that low oxygen=lower performance? The chronically low Hb in many minors does cause problems over time and I suspect that over the years, it can also lead to pulmonary hypertension if the Hb is low enough.
Supplements can help. B-complex, vitamins D & E, folic acid and L-carntine can all help. I'm finding that minors who get serious about supplementing are seeing positive results. I cannot say that any thal minor can do everything physically that they wish, but I do think that all can improve their health by paying special attention to diet and nutrition.