I'll address each paragraph:
1. That exact pairing has started and won 2 UCL Finals, they are in no way similiar in terms of how they play, exact opposites actually. Vallejo's only concern is not talent but rather injury. Have a look at his performances in Germany and the accolades of not only his manager but other pundits in the league.
Not much of a concern in single big games, as Ramos tends to play those with 100% concentration for the whole 90 minutes and that cuts a lot of his mistakes out. Fully focused, Ramos is one of the 3-4 best defenders in the world
Ramos and Varane are similar in that they're both instinct types who rely a lot on their athleticism rather than reading the situation and making sound decisions based on that. They're both very mistake-prone, and they make similar types of mistakes(in Ramos case it's mostly down to lack of concentration), and unless Ramos is in his big-game mode, they won't cover for each other
Both of them have been better playing next to Pepe, a true defensive leader who's very good at reading and anticipating play, and doesn't make rash decisions
Vallejo's question marks is down to entirely to his age. Being 3rd choice for us is different than playing for Eintracht or Spain U21
2. Modric is 32 and despite Isco's performances in the diamond, if Bale is fit he is playing. Also majority of Isco's starting roles came in Modric's position, with Modric being 32 and coming into a world cup year he will be rested even more.
Isco had to play in midfield because Bale was untouchable. Cris, Benzema and Bale are no longer untouchable, and Isco is waaay better playing AM than CM, as he just showed this season. But yeah, it's more wishful thinking on my part than anything at the moment. We'll just have to wait and see
3. Kovacic is often and repeatedly mentioned by our coaching staff as having one of the highest footballing iq's on our squad. Look at his performances when Casemiro was injured, he was superb, especially against Atletico and Barcelona. he has huge potential. He is also one of the youngest players, he has plenty of time, see Isco for reference.
And it always struck me as them sticking out for him. He's a good player and still young, decent to good at just about everything, but only really great at one thing. Ceballos is better at several things. He's just better than Kovacic imho
Also, funny that you mention his performances in Casemiro's position. And true as well, that's where he's looked best for us. Maybe he could turn into a WC DM...
4. No one not the media, not the staff, not even the rag tabloids have ever mentioned Kovacic leaving. It will be James if it is anybody.
Yeah, that's just what i think will happen, not something that's actually happening. I might be way off here. As i said, i don't particularly care either way though. He's good, but i don't see him displacing Modric in 2 years time. I can see Ceballos doing that instead