Test cricket gives all 11 (22) players chance to come into game at some stage. cricket, by nature is a slow & long game, it needs time for every player to show case their skills. Hence, in Test there are several come back option; even tight draws are exciting.
First, T20 does given enough chances to every player in a game; then the duration is too short to bring any tactics or combination. It's good fun indeed, but a bit shallow - often becomes boring & predictable.
By nature, cricket is one of the most complex game to understand. In terms of understanding the finer skills of the game, it's probably most complex; add to that the 5 day duration - it's really surprising that still Test cricket has so much following. This is 21st century, when even FB is becoming slowpo to Tweeter & snapchat, therefore excepting too many people in ground for 38 hours (exactly a full working week considering lunch, tea breaks & extended hours) is foolish; but lots of people still actually follow Test cricket via tv or net, even radio.
The fun of T20 is in it's actions (batting, bowling, fielding, cheer-leading ...), therefore one has to be attached with the game & see it. A good Test is like a thriller movie series - slow but mentally demanding, which is difficult for most common people.
ODI is probably in between both - ODI is the most tactical game; often nail biter as well. Most common people can't see the difference between ODI & T20, therefore popular belief is that both should merge (basically close ODI chapter), but both are totally different game. It's not surprising to me that historically, the correlation between Test & ODI ranking is much stronger than that of ODI & T20, though later 2 seems to be similar game.
All 3 formats should co-exist - at highest hierarchy, first 2 at National level, 3rd one at club/franchise level. The skills of this game is extremely complex and perfectionist job; learned from long hours in game condition - without Test (First Class) cricket, those skills set can't be learned, mastered - game will eventually die, if T20 takes over too much.
