If cricket is restricted to only T20, and further seen as just run-scoring fest with rules favoring batsmen, negating everything (spin, swing, seam), then it will no more remain as "Cricket" it will be evolved into a different game! If people (of fast world) want to miss the "art" of cricket like skill of bowling & batting against spin, swing and seam, then its their wish. For that to happen the match has to go on for some length. If the ball doesn't get old and the surface is not scrubbed, then no way the quality of spin bowling becomes visible (and also batsmen playing against it) If cricket is not played in overcast condition/green pitch, then quality swing bowling will not be seen and also the batsmen combating it & then carrying his innings after 2 hours (or first day) will not be experienced!
The reason for failure of 1 day cricket is this reason only. 1 day cricket was giving the flavor of test cricket in 1 day itself (especially when it was played in day-time) Ball getting old, not much batting friendly rules meant that the matches were interesting and not like extended T20.
The reason for poor turnout for test matches & ODI matches (from financial point of view) is because of wide coverage for T20. If T20 was not there, people would have continued to watch Test & ODI matches with same interest (may not be with similar financial outburst). Teams like India would not have improved their fielding skills & athleticism (like 90s) and remained a team winning purely based on raw skills (That is the reason a team like Pakistan because of lesser financial throughput somewhat plays like as in 90s) While players from Australia & other countries have basic facilities for sports in general would remain the same always. Because of financial turnaround the Indian players & system got money to improve on everything at every levels.
Hence cricket will be there, but may get evolved into totally different sport. If ICC plans to expand the market aggressively to various nations, then T20 will be the future. Otherwise if they continue slowly with the traditional way, then ODIs & Tests will exist for some more years, though we can't predict what happens at 2050, because its a long way to go (Somebody during 1970s & 80s wouldn't have dreamed about today's T20 and the kind of shots batsmen are playing today. They got clue only during 90s. Similarly we may get clue about anything only before 10 or 20 years prior)