Indians are required to renounce their Indian citizenship after obtaining another citizenship.
What happens if someone doesn't do so?
I mean if my Indian passport has anyhow expired, and I never ever plan to travel to India again, or use my Indian citizenship in any way, does it make any sense to pay 185 euros for this procedure? Can I just simply claim on official forms that I'm not an Indian citizen after my Indian passport expired, even if I don't officially renounce it?
The Indian Citizenship Act, 1955 §8 requires it, so from a legal perspective, you must do it. That law does not specify the consequences, and there isn't a clear limit on the possible punishment. The government does invite you to inquire about your penalty, saying
The Indian Citizenship Act, 1955, does not allow dual citizenship. Holding Indian passport/acquiring Indian passport/travelling on Indian passport after acquisition of foreign citizenship constitutes an offence under the Indian Passport Act, 1967, and attracts penalties. The Government of India has prescribed imposition of penalty on a graded scale, depending on number of trips made on Indian passport after acquiring foreign nationality, for the violation of Passport Rules and retention of Indian Passport for more than 3 years after acquiring of foreign nationality.
§12 of the Passport Act spells out the penalties (up to 5 years in prison and ₹50,000. If you can avoid being in illegal possession of an Indian passport, you might avoid prosecution under the act (though they probably cannot get you extradited to enforce the penalty). They certainly have the power to revoke your passport, but no provision of the law obligates you to formally apply for a passport revocation when you believe that your passport should be revoked.
Unless you officially renounce your citizenship, do the paperwork, and officially surrender your passport, the government can reasonably suspect that you are illegally in possession of an illegal passport, so they are empowered to investigate, including searching and seizing. However, if you have zero interest in India, that may not matter.
Depending on which is your second nationality, if you don't present the official papers proving that you requested renounciation and then confirming that your renounciation was granted, your second nationality can get revoked
Example: The Netherlands
