mythic isn't just boosted numbers a lot of the encounters i've tried and seen videos for seem like they have entirely different mechanics, at least the tactics that work for normal and heroic do not work for all mythic encounters, it isn't just a number tweak, its also a mechanics tweak. for most if not all of them have some extra mechanic or feature that has to be worked into your tactic.
in the past there was only 1 version of the content so if you blitzed through it you were stuck farming that faceroll raid until they released a whole new tier.
the challenge is always going to be a challenge for the sake of being a challenge. because its inherently designed that way. we have 13 classes with a myriad of abilities and so they create encounters where the best use of those abilities are required to down the boss. intimate knowledge of your character is required to progress. the difficulty today mostly comes from dodging one shot mechanics and dps checks. everything else has been the same for years, dodge fire, interrupt casts, kill adds, kite something, tank swaps. every single boss fight is a remix of mechanics some old some new.
but that is what new tiers of raids are anyway, just number tweaks, they take your raids theoretical output from one tier and create boss encounters around that for the next tier.
normal en/tov are about the same difficulty as raid finder/normal nighthold, heroic nighthold is about on par with mythic en. the raids that drop the same ilvl have roughly the same difficulty and requirements.
heroic ToS will probably be about on par with mythic nighthold, perhaps not mechanically but number wise, it will be. they will likely also only be separated by 5-10 ilvls on the base drops.
I never did any challenge modes either, But i missed WoD because ppl were constantly complaining about various things, I like mythic + because you have a scaling challenge that requires only 5 ppl. you can farm them if you want or you can just run your 1x 15 for the week and be done with them. I think its great that 5 man groups can progress, every form of progression in the game is a funnel, the idea is to tune players and urge them into the hardest difficulty of raiding, I bet blizzard want lots and lots of ppl raiding mythic, its mostly time constraints that make it hard for ppl to commit to.
most of the content in the game is designed to eventually funnel you into mythic raiding, you will eventually have enough gear to try it, its just up to you if you want to take the plunge or not. noone is going to force you to raid mythic, but it is there for ppl who want to try it. all the content before that is there to get you into it.
but you don't have to progress through mythic, you can progress through just normal, or just heroic, or just the raid finders, each difficulty is either its own stand alone progression or another means of getting loot for the difficulty you are mainly focused on.
for example, clearing normal nighthold gives you gear that will make progress in heroic easier, but it will also give you the gear you need to start progress in normal ToS when that releases, sure you probably won't need the whole raid to have 4 piece set bonuses to clear normal tos, but it will help.
you can either progress through the difficulties lfr>normal>heroic>mythic or you can progress through just the tiers, tier1 normal> tier 2 normal>tier 3 normal.
the choice is really up to you, there being multiple difficulties is hardly more of an imagination stretch as killing bosses every week like some sort of reoccurring dream.