I think you're right, I just found this:

Nearly impossible
The switch to natural grass, for Lucas Oil, seems nearly impossible. Unlike other stadiums, Lucas Oil is not equipped with the irrigation and drainage required to grow and sustain natural grass. The implementation of the necessary irrigation and drainage systems would be very costly for the stadium.

The stadium is used year round with many events from college basket ball to conventions. They are going to put a pool in there for a swim event this year. The. Colts have a possibility of 12 dates every year. If they built the stadium outside the interstate loop they might have had the space to have a mobile turf like Arizona but it would not be practicle for the covention / non sports related events that go one every week in Indy.

Titans fans rarely give the Colts any props. It is what it is. This defense is solid, with backups playing well when spelling a starter. We have excellent scouts who find football players, and we have good coaches who teach them the pro game. The future is bright.

not unless they do a major renovation to add drainage

Grow lights, and heating. Opening the roof for sun light during winter isnt an option.

The city needs the facility making money year round to justify its construction.

Well I was going to write an note about how cheap Irsay and Indianapolis were to not spend the 'millions' to change to natural gas. Decided to look it up. There are estimates of one billion (yes billion) dollars to change to natural gas. Artificial grass it is.

They're not switching to grass. Good God...

No, it's not possible. Arizona does it because the entire field is moved outside on a giant rack, that is not possible here. But a safer field is needed ASAP!

Then they need to improve or recreate artificial turf. Sounds like a job for Purdue U.