2015 cars are significantly more aerodynamic than the 2014 counterparts, and the powertrains especially are beefier as well, they simply cannot be compared.
If you look into how the engine development tokens had a special rule implemented for Honda, where out of the 32 tokens available to all the manufacturers, they would be allowed to use the average amount of tokens left after the other three engine manufacturers have done with their first homologation step. Which means if Mercedes, Ferrari and Renault have used up a bunch of tokens and came to the party with beefed up power units with few tokens left, then that would mean Honda would get shafted pretty hard since they will have less to work with in terms of upgrading through out the season. And that is exactly what happened where everyone else showed up with like 10 tokens left and Mercedes showed up with
7, Honda had to negotiate with FIA to have 9 tokens, which automatically and drastically reduced their chance of doing any major upgrades to the unit other than do some minor reliability fiddling every now and then... even more difficult considering for this season they have to make 4 power units last the entire year.
So unless Honda showed up on the grid with a class leading engine with solid performance they had no chance and were inevitably going to get pummeled, even though they had one year of extra development time they could not deliver that sparkly engine that could have saved them from this plight. Now all they can do is hope the remaining engines don't break. It's all pretty dire like some sort of Game of Thrones episode when you think about it.