Indian domestic air fares are still very low when compared to fares in other countries. Airlines in India have higher fixed costs than in many other countries. So 12k for BOM-DEL roundtrip may be ok, due to inflation.

Akasa has entered the market, and G8 may be on its way out. In most sectors, G8 does not fly and in the few they did, pricing wont be affected much. Indigo is sometimes the only airline in many sectors, and in those sectors, the airfares are still quite reasonable.