Photos photolibrary can't open in MacOS Ventura anymore

Photos (and iPhoto) was never backward compatible. Any library opened with a specific version of Photos is not compatible with earlier versions. It's always been that way.

So if you want tow Macs to use the same library they both will have to be running the same system and Photos versions. OR use iCloud Photos.

"I guess it would be very easy to make it backwards compatible, as I said. Or let the new app be installed by older systems. I mean, Ventura is not that OLDER system."

That is not so easy. The Photos.app is a part of the system. And the Photos.app on Sonoma is different from Photos on Ventura, also the Photos Library. Apple would have to update Photos on Ventura to enable it to read Sonoma Photos Libraries. The only tool Apple is offering to access our Photos Library from different platforms and from different system versions is iCloud Photos. The iCloud syncing helps us to work with one library on an iPhone, and iPad, a Mac, and from several different system versions. But it requires a subscription for additional iCloud Storage. For me it is worth it. I can work with my photos and videos on the older Macs with macOS 10.14 Mojave, on Ventura, and Sonoma, on my iPad and my iPhone and my Apple TV or in any web browser on any device at www.icloud.com .

I understand that you do not want to use iCloud Photos.

But did you not keep a backup copy of your Photos Library, before you upgraded to Sonoma? Use your Ventura backup copy to be able to access your photos on your older Mac.

Either restore the old library from a backup to Ventura.

Or upload to iCloud Photos from Sonoma and then in Ventura use that library via iCloud (I don't use iCloud Photos but I think it should work ...although it takes time with such a large library...).

No, it's not really stupid. Photos is a database, sort of like Excel. A new version has new columns of parameters to describe the pictures and to help organize them. Of course, the old database program won't understand the new columns. Thankfully, the new version converts the old data to fit the new database set. And luckily, as Matti Haveri has said, an iCloud sync with the Sonoma machine can help fix up new data to fit old databases. Just make a new empty Library on the Ventura machine, and it will populate (after a time) with all the pictures.

Be sure to back up what you have before messing around with it!

Why does Apple always make simple things so complex ??? Just to push you tu use iCLoud ? Aperture was sooooo good compared to Photos. What a mess !

Really disapointed by the way they handle customers using them for a long long time...

Apple should give us the tools they use for iCloud to enable us to do it locally if we want. I am very sorry to see how little they consider their customers needs.