First of all, past performance is not indicative of future results.
Second, it's obviously much easier to double your user base from 2 to 4, than from 10 million to 20. New languages tend to have a fast initial growth, but then the curve flattens and no significant change comes after. Java hasn't been on a decline in the absolute sense, the whole market has just grown and has more players now, so the same mindshare gives less of a percentage now. Nonetheless, no language has come even close to the top 3 languages (js, python, java) in popularity that were released in the last 20 years, so what makes you think that Rust will be the one that avoids the flattening of its growth?
I'd like some references of "Java mindshare is going down fast".
Sure, lots of new project/code uses languages like Go these days. But Java is still very dominant, especially at companies like Amazon. It currently sits ar #3 on TIOBE.
Second, it's obviously much easier to double your user base from 2 to 4, than from 10 million to 20. New languages tend to have a fast initial growth, but then the curve flattens and no significant change comes after. Java hasn't been on a decline in the absolute sense, the whole market has just grown and has more players now, so the same mindshare gives less of a percentage now. Nonetheless, no language has come even close to the top 3 languages (js, python, java) in popularity that were released in the last 20 years, so what makes you think that Rust will be the one that avoids the flattening of its growth?