The reason I suggested Marylebone is that it is a very busy line in an area where bad air quality is a serious problem. Obviously a detailed design would need to be made, taking into account the challenges such as tunnels and the overlap with the tube (and loads of others). As far as I'm aware, no serious attempt has ever been made to do that for these busy lines. In any case the point wasn't the specific scheme, but rather that schemes that should be obvious haven't been progressed, whether the obvious scheme is in London, Birmingham or elsewhere.
I didn't mention Birmingham as I don't really know anything about it, other than that New Street station is very unpleasant due to the diesel fumes.
Also, you could have made your point without immediately being pretty rude against someone you've never met!
Marylebone isn't "very busy" in the scheme of things - in a normal hour it sends out 9 trains (looking between 3pm and 4pm today). The reason it hasn't been higher up the list of "to do" for NR or its predecessors is there have always been other, busier termini within London. Paddington was sending 50% more than Marylebone, virtually all diesel until the GWML was electrified.
Once out of London it runs through much less densely populated areas until it gets to Oxford or Birmingham - contrast this with the GWML or Midland Mainline.
All that's before you get to the challenges of bridges, shared infrastructure with London Underground etc - so it's not a "no brainer" - it would have been considered against many other schemes and the benefits of those were higher. And if you were looking at the Chiltern line the bigger gains would be to address the Birmingham area - if you take Moor Street it sends out 11 trains in a normal hour - more than Marylebone, all diesel, all running stop / start services through the West Mids. And it is stop/start services which cause more pollution than longer distance services (of the kind you see out of Marylebone) because the fuel consumption is higher on stop / start services.