Average monthly data usage for a family hovers around 400-800GB even if you stream 4K content on multiple devices. Even a power user who downloads content/games won't be able to cross the standard FUP limit of 3.3TB even if one is hosting a file server/
NAS. Increasing the FUP limit makes very little sense for ISPs on all their plans, they would have a much more accurate data of how much data is consumed by their customers.
BSNL is known to have very bad routing and connection quality, goes down quite often or there is speed drop. Even if one goes for a 500 Mbps plan from BSNL(which no one in their right minds would do unless they don't have any other option), they won't be able to hit the given FUP limit even if they want to.
Afaik excitel doesn't have any FUP but it comes with a cost of poor internet, bad routing etc. If one just wants crazy download speeds for less money, sure excitel is the way to go but most of the users who are students, families, working professionals, content creators or elderly citizens would value connection quality and uptime over higher FUP. Most of them won't even cross 1TB of data usage.
We all have seen the abuse of free and unlimited internet in 2016 when Jio was launched. Sure there were good sides to it but providing too much data would just increase the misuse of it at some point. In my opinion ISPs should launch a booster pack for extra data let's say 1TB for a price if someone crosses the 3.3TB FUP limit.