The time-span in question is exactly the last 15 months. If you take the last 18 months, suddenly you add very convincing wins in NAC3 and HC3Liereey or Hera. Results that are starting to be from quite long span now just don't let you draw any other conclusion.
In these 15 months though, there have been 7 major tournaments: 4 RBWs, 1 KotD, 1 HC, 1 DMWC, 0 NACs.
I think the fact that Empire Wars was so much more present during the last year is a big factor of Viper's perceived falldown. While his performances in KotD and HC underlined that he is not that dominant as in the past, the performances that really put Hera and Lierrey in front of him definitely have been those in the RBWs and I think if we had only 2 RBWs but additionally a NAC 4 and NAC 5 or some other mixed-maps RM tournaments, the perception would be leaning more towards Viper than it does atm.
Also, keep in mind that there is a bias to over-interpret single results of Viper because of his history. Lierrey was beaten very convincingly by Hera in HC4, much more convincingly than Hera beat Viper with a home map advantage later on. But this sub-standard performance of Lierrey didn't get half the attention of Viper's loss. Lierrey also lost twice to Daut in S-Tier-tournaments. I think it's safe to say that, if the same happened to Viper, it would have been a much, much bigger deal. (Actually, Viper's loss to Villese in RBW 3 is quite comparable and it got way more buzz.)
Also the fact that Hera and Lierrey were matched against each other three times very early on in the major tournaments (Ro8 in 3 consecutive tournaments) led to less opportunities for them to fail against Viper or an inferior player. That's not an argument in favor of Viper, but just puts us in a situation where we lack information to make a proper judgement.
I think, it is very, very difficult at the moment to compare them. I think, it's quite clearly the case that Lierrey is the best in early game and Viper is the best in late game. Also Viper is the least consistent of the three in early game which was the main reason for his bad results. With early game being more important than late game, that will probably put Hera and Lierrey in a better spot than Viper, but Viper might still be able to put out the best peak performances and could be seen as the favorite in mixed RM-settings with a map pool that is not particularly open.
It could be the case that, depending on the settings, Viper or Lierrey will be #1 and independently of the settings, Hera will be a very close #2. (And I'd say that the difference between #1 and Hera tends to be smaller than the difference between Hera and #3, so that Hera might be the general #1 without being the #1 in any specific setting.)
Then again, it also depends on how much you take DM into account. Lierrey and Hera don't even compete there, Viper dominates there. If you'd rank EW-DM-RM all similarly important or DM at least closely important to the other 2 modes, then Viper would be the clear #1. (But I think, that is not the importance that people do put on DM. Otherwise, Nili could be seen as a Top 10 player and Tatoh would still be the #2.)