Saying they weren't much of a step up from heroic is just downright incorrect though. And multiple bosses of previous difficulties did in fact have wipe counts in that range. Using Method as our example here, literally the World First guild for every single one of the tiers I'll be using for examples, and citing two sources from
before your supposed sudden increase in difficulty during Trial of Valor:
Blackrock Foundry:
http://i.imgur.com/z8P1vAY.png
Hellfire Citadel:
http://i.imgur.com/DRPPPX3.png
Now obviously the raids have different quantities of bosses, so we'll use the average wipes per boss number to do the math on these raids.
BRF: 65.4 wipes/boss
HFC: 75.5 wipes/boss
Moving on to the current expansion, or after the supposed difficulty jump:
Uldir:
https://twitter.com/methodshades/sta...342976?lang=en
Battle for Dazar'alor:
https://twitter.com/methodshades/sta...51788326334466
The Eternal Palace:
https://twitter.com/methodshades/sta...505473?lang=en
Leaving us with the following wipe/boss averages:
Uldir: 83.3 wipes/boss
BoD: 53.5 wipes/boss
EP: 79.6 wipes/boss
Overall the numbers are fairly comparable, with the easiest raid of the bunch being one from the current expansion. The only really large jump in the numbers was for Tomb of Sargeras, featured here:
http://i.imgur.com/9ndVMNW.jpg
With an average of over 137 wipes per boss, this one particular raid takes the cake for now because of the difficulty that mythic Kil'jaeden provided. You're objectively wrong; the difficulty curve between the raids is relatively close when you actually take the time to look at the numbers. Mythic raids are generally steamrolled until you hit a few bosses that are meant to be giant roadblocks; in this case, you're looking at Blast Furnace and Blackrock for BRF, Xhul'horrac and Archimonde in particular for HFC, Mythrax and G'huun for Uldir, Jaina being the only one for BfD, and Zaqul/Azshara for EP. There's nothing in particular that suddenly swung to make mythic somehow become much more difficult at any point around Trial of Valor's release; the fights are all designed with mostly similar mechanics and higher number tuning with the exception of last bosses generally having an extra phase or particularly powerful new mechanic.