Well, yes and no. There is still a far bigger audience for test cricket than there is for T20 cricket, especially in traditional test-loving countries like England and Australia who are able to offer financially attractive central contract to their test players, and I don't see that changing anytime soon.
But T20 has had a bigger impact in some of the poorer test playing nations, like the West Indies, who have lost a lot of their players to T20 cricket because they are financially outmuscled by the organised crime syndicates that run the IPL, and as a result the quality of their test team has fallen so far that they are barely test standard at all.
However, this isn't limited to the west indies. The quality of test match cricket has plummeted world-wide over the past 5 years, and this is entirely due to a contraction in the players willing to focus on red ball skills.
Young players have been taught white ball skills in place of red ball skills for a decade or more now; it should therefore have come as no great surprise when the standard of red ball cricket started to decline as it did a few years back.