I'm repeating my post from the other thread. I'd say around another 20-25 years. Too complicated and expensive to play and doesn't make any money outside big 3. Raw talent not as useful in Test cricket as other formats and sports. When the contest is too one sided it's agonising to watch over a month the other team being thrashed. Underdog factor is significantly reduced in Test cricket as luck doesn't play as big a part. Will never take off among casuals as it's too long to follow. Number of competitive test teams are very few, maybe 5 or 6 if you want to stretch the term competitive. Too many negatives, not sure how it can survive.
I'd confidently say test cricket is dead in countries like SL, WI, PAK(also due to other reasons). NZ and SA(it has other issues anyway which puts its long term future in question) get minimal interest in test cricket from majority of sports fans in their countries. India is unpredictable, it doesn't get big crowds for tests but their players like Kohli still have star power which might draw the crowds now and again. Basically test cricket is healthy in only England and Australia. I read that the host boards actually lose money when they host a test series between non big 3 countries, it's actually a sad state of affairs, cricket is actually one of the most expensive games to play and I'm not sure how cricketers from developing countries will be able to compete with developed countries like AUS & ENG in an expensive game like Test cricket as the gap is widening between the cricketers from the rich boards and the poor ones. In 20 years I predict Ashes to be the only form of test cricket to be played.