I can't follow your analogy because for me it looks like this: I have a kid (Eternal) that is going to school soon, so I know the associated costs will go up beyond what I can sustain. How will having a second baby (Stanard), which has its own, admittedly lower, cost base reduce my overall child raising costs? The only way this is possible, if I decide to abandon my older kid going to school (Eternal), which is a rather cruel analogy, but you started it! :P
Maybe I have a flaw in my logic, so happy to be convinced otherwise, but that is causing the core issue I see a lot of people fearing: Their favourite champion will rotate into an abandoned format.
You, i.e. Riot, wisely decided to tap into the investment done into making champions into characters that players can and want to get attached to for gameplay, presentation, and/or lore reasons, but the downside is that LoR players now care a lot more about champions in LoR than MtG players about certain Planeswalkers or Hearthstone players about certain Legendaries.
I'm hoping to be wrong, but I see a significant amount of players being very disappointed if their favourite champions gets rotated, and these are most likely people who not only have emotional but also financial investment into that champion; unless you manage to make both formats feel equally supported, but then rotation won't help with the QA and balancing issues an ever increasing card pool causes? And just to be clear, I don't envy you at all. This is a very hard decision because an ever increasing card pool truly comes with ever increasing dev resources needed to maintain the same level of QA and balance.