how is the yeild higher than international trade routes?
Internal gives food, and production to 2 cities...
International gives science and energy, which benefits your entire nation.
And, energy is only worth half of a hammer or food. And, internal TRs give you more of those double-value dots than you get energy from external TRs. And, hammers and food turn into science, but science can't turn into hammers without additional committed hammers and food. And, this "entire nation" distinction.. I just... what? I actually can only spend my energy in one place at a time. I actually can only apply new techs in one city at a time by
building things in those cities with hammers, and having the population to work the tiles that get added perks, both of which, you may have noticed, require
hammers and food, the yield of internal TRs. The distinction is pointless. It all feeds into everything else. Your empire is an organism.
Higher yield is better for the organism. Internal TRs are worth more than external, but you, yes, obviously, need
some external to accelerate unlocking new buildings to build when your OP cities have already finished all the available ones 20 turns after being founded. Yes you need a balance. No the yields aren't balanced.
It doesn't matter that some internal TR options will flatline. Because, just move to the good ones - the game puts no real cost on this, so you are
guaranteed enough good internal TR options to make their performance virtually optimal all the time.
"Hey, the game lets you choose 24 of 26 free bonuses all game, but it's
ok because at any given moment 2 of the choices will be bad"? No. It's not ok. It's pointless because you're not really making a choice anymore.
Just take out iTRs and reduce hammer costs across the board for the literal same effect.