You r looking into just the overall figures, go deep into it and u will see Airtel still leads in 4G segment and particularly has strong base in customer segments which pay more for the services/higher plans thus ARPU of Airtel is quite good...also their customer base in mass audience is also quite good just somwhat behind Jio but still large volume helps earn good revenue here even though ARPU is lower than what they get from urban/premium audience.
Lastly Jio may have caused disruption and brought prices lower, infact i even agree that pre 2016 prices of data/calls were little bit on higher side but remember Jio/Reliance is also here to do business and not charity....initially giving freebies to attract subscribers and thereby causung rival brand customer base go down surely lead to them bevoming No. 1 but this cannot.be a long term business strategy....i had said few years back too that wait a little soon Jio will also increase pricing + reduce freebies to arrive at sustainable business model and so it happened.
Airtel was able to sustain all the hardships bcoz they deployed a smart strategy of not going the Jio way and causing financial losses to them...instead they reduced prices by smaller margin ad gave some freebies thereby letting go off customers who were wanting all things for free so Airtel managed to earn decent revenue many premium and other regular customers stayed with them...some % of these regular customers may have used Jio as primary SIM and airtel as secondary but did not completely shift to Jio.
Later on as many people noticed that there were lot of issues being faced quality of service which Jio offered as compared to Airtel so they started shifting back and as news spread even many new subscribers chose airtel as 1st option.
Now that Jio has also somewhat started raising prices in telecom sector hence Airtel has also shown more improvements in ARPU / Revenue thru similar increase in plan/service charges. If Jio increases prices further i feel more people will shift to Airtel.
Just like this Disney-Star will be able to counter Viacom18/Jio if they take right decisions which i am sure they would and already started work on the same sinve last 6 months or so