Exactly this. More than Rohit getting better, I would love to have Jadeja getting better in his batting so that we have the bolded one.
See this strikes me as the job Jadeja was always supposed to do, but then he turned into a bowler.
Young Jadeja was a legit middle order batsman. Batted middle order for India U-19, batted middle order domestically, and even batted at 4 for RR in the IPL. He was then forced into the number 7 slot in the ODI side, where he sucked, which was no surprise to anyone.
Since then he has completely re-vamped his batting. His stance is more open now. He clears the front leg more. He's improved his power-hitting and slogging out of sight, but at the expense of a proper defensive game.
Current-day Jadeja is nowhere near what a Test Number 6 should be like. He is perfect for playing jammy counter-attacking innings/downhill skiing at 8 in Tests and ideal for end-of-innings shenanigans in limited-overs cricket. He possess enough batting nous to be useful even in tough conditions - as we saw this series and in England. But he would be batting far out of his depth at number 6.
Maybe if he re-worked his batting again, he might be good enough. I doubt he ever possessed the ability to bat in India's top 7 tbh. Even if he went back to his old batting style and maximised his ability in that department, he's still probably won't be a 40-average calibre Test batsman. On top of that, he can't afford to let his bowling slip again. Peak Jadeja is, at best, a second spinner away from home. Right now what we have is a capable second spinner and a handy lower order batsman. If he works on his batting, it will take away from his bowling, and what we will have is a not-quite-good enough middle order batsman and a not-quite-good-enough 5th bowler. Basically we will have another Stuart Binny.
Lokesh Rahul is indeed an opener. It's that funky ICC logic coming into play here. He's not going to displace any of the Top 3 unless one of them is injured/in a terrible run of form, so might as well play him at 6. It's what we did with VVS - he was a number 3 domestically but wasn't ever going to displace Dravid from that slot, even after the 281. Worked out just fine.