Say next expac theres a tforge cap of 1200.
Heroic raider get 1170
Mythic gets 1185 as a base.
Heroic raider is lucky and tforges bis trinket up to 1190, Mythic raider never even sees a warforge. End the patch has the heroic player ahead in terms of that slot,, solely because of luck.
By reimplementing the system that was in place in MoP (maybe even in WoD idk, didn't play they) allow for 2 +5ilvl upgrades per week.
1185 to 1200 is 15, so 1.5 weeks per item. If you assume around 154 days between content patches, that's 22 weeks, enough for 14 fully upgraded items. Factor in something is going to warforge / tforge and you should be at full 1200 by end of content patch if you consistently raid mythic. Then when the next content patch hits, old items are locked to their previous cap (ie you can't upgrade a convergence of fates to a TOS or even Argus ilvl)