You can only use iPhoto on a new Mac, if you already have a copy of iPhoto 9.6.1. Only iPhoto 9.6.1 can run on a Mac with macOS 10.10.3 or newer installed.
To get iPhoto 9.6.1 installed, check your Purchases history at the Mac App Store. If iPhoto is in your Purchase History you can move any older iPhoto.app to the Trash (only the iPhoto.app, not the iPhoto Library) and then try to download iPhoto from the Purchases history at the AppStore.
You can use iPhoto to open your old iPhoto Libraries, but iPhoto cannot open Photos Libraries. You would have to export the photos from Photos and to import them to iPhoto.
iPhoto is essentially dead. It has not been updated since 2015. And it will no longer run on the next version of MacOS X, macOS 10.15 Catalina. If you want to use iPhoto, you must not upgrade your Mac to the new system, when Catalina will be released next month.
My whole system want down the drain
In what way? It might be easier to fix this problem than going back to iPhoto. It will be a dead end.