Is there any good estimation of when, in 2019, the inflation rate of XMR will become inferior to the inflation rate of BTC?
I assume you are asking about the supply rate as it's impossible to foresee the inflation based on demand (or I should re-read Milton Friedman). In that case, here is the answer:

Note the supply rate will be similar from 2019 to 2027. You can read this very interesting reddit post to get more info:
https://www.reddit.com/r/Monero/comments/512kwh/useful_for_learning_about_monero_coin_emission/