Most people have pointed out what is probably the most glaring point that I was going to mention, and that's simply that Sm4sh's competitive scene will lie largely in how the next instalment of the game is received by the crowd. If Smash 5 is received like Brawl was (let's hope not though), then there's no question that Sm4sh's competitive scene will last longer, and hence, evolve further.
When I think about Melee, I think about how much it has evolved technically. Some of this is no doubt due to Melee's unique physics allowing different techs to occur, but it is also due to the sheer amount of time that people had playing it, and the mastery that resulted. Playing a game competitively from near the end of 2001 until the latter part of 2014 is a VERY long time (and there are many who still play Melee), so it's quite natural that Melee ended up the way that it did. Although there may have been gaps in the earlier part of 2008 where people were on the Brawl Hype Train, for most Melee players though, it really didn't last long before they reverted.
Of course, there will always be a chunk of the community who will undoubtedly move onto the next instalment for other reasons (new bells & whistles, new characters, stages, etc) as what happened in Brawl, but if the game is not deemed benign from a competitive standpoint, then the competitive scene will remain within the previous instalment. Sm4sh has the potential for this, both largely because it has a lot of new players involved in it, and it doesn't seem to be generating nearly as much negativity as Brawl did, and some of that negativity that it did receive has been pacified by balancing patches. However, whether it will reach Melee levels is uncertain, and something that only time can tell us. The reality is, we have no way of knowing how Sm4sh's meta-game can evolve great Smash 5 will be (or if there even will be one). So until then, yes, there will be a competitive scene in Sm4sh that will continue to flourish and push the meta-game as much as it possibly can.