My guess is that if Lewandowski leaves Bayern Munich, the Bayern Munich management will consider Ronaldo as Lewandowski's replacement. I don't think they will put Lewandowski and Ronaldo in the squad at the same time. This will ruin their match strategy or formation. If Lewandowski wants to leave the team, Bayern Munich is trying their best to keep Lewandowski in the team. We will have to wait a few more days to see if the Bayern Munich management will sign Ronaldo.
I agree with you that Lewandowski will probably leave Bayern Munich this summer. A few weeks ago Bayern was very clear that there is no chance at all that they will let Lewandowski go now but things have changed in the last few weeks with the signing of Sadio Mane and also other stuff that happened. Now the Bayern officials are not ruling it out categorically anymore that Lewandowski will leave the club. I personally hope that they let him go if Barcelona is making an offer of 45 or even 50 Million € for him.
Even if Lewandowski is leaving though there is not chance at all that Bayern will sign Ronaldo. They are very reasonable about how they are spending their money and they would never pay a salary of 30 Million € per year or more for a 37 year old player.