Why can't stadium projects ever go according to plan? AC Milan was supposed to move out and build their own stadium in the Portello region by the 2018-19 season. While this was going on Inter was going to renovate the San Siro (probably decreasing its capacity as well) and look towards buying it out. This way both clubs would have their own stadiums and they wouldn't have to pay rent to the Milan city council (currently their deal is a lease to use the stadium until 2030 at a combined rent of €9m per year).
Unfortunately it seems that Milan has seen its relationship with Fondazione Fiera, the company that owns the land the club wants to build on, deteriorate to the point that negotiations have essentially come to a halt. This in turn lead to an announcement by Milan's president Silvio Berlusconi stating that he believes Milan will stay at the San Siro instead of pushing ahead with their new stadium project.
This news leaves Thohir in an awkward position. Milan's staying at the San Siro should free Inter from any obligations to foot the bill for future renovations to the San Siro (besides the already agreed upon updates for the 2016 Champions League final which will be paid for by both teams). But now, it is unclear as to what the future holds for the long-serving stadium.
One possible option would be for the two clubs to basically switch proposals. Inter would leave the San Siro and build a new stadium elsewhere while Milan stayed and did whatever renovations they saw fit. This idea had previously been discussed when Thohir first joined the club and the Rho district had been proposed, but Milan moved faster and the idea was shelved. Now it might come back into play.
The second option would be for the status quo to remain. Both sides would stay at the San Siro and work togehter to renovate the stadium. This would reduce the costs but would also leave both team still paying rent to the Milan city council and losing out on a portion of match day revenue (as well as revenue from other events that occur at the stadium).
The third option would simply see Milan get their act together and follow through on their proposal. Berlusconi claims that both he and soon to be new owner Mr Bee changed their mind about a new stadium because they suddenly "really like it [the San Siro]", and think that it is the best solution but it makes more sense that they're just saying this to try to save face because this change is occurring due to failed negotiations. An impasse in negotiations can usually be resolved, but this is Milan we're talking about. After failing to secure Kondogbia, Jackson Martinez, and Carlo Ancelotti, maybe Berlusconi is just losing his touch...
Whatever the case may be, we can just hope that this situation gets resolved in a way that is beneficial to Inter. I think both clubs would prefer not to keep the current arrangement of sharing the San Siro and paying rent to the Milan city council, but we will have to see which option ends up occurring.