Several things going on here.
4+ photos sent via iMessage are sent through a link to iCloud. 3 or less photos sent are sent as the files themselves. This is a requirement with "Messages in iCloud".
There are several benefits and features to this:
Features :
1. Gives the recipient the ability down download the photos anywhere. They can use the link on a PC.
2. Individual photos can be downloaded from the link. So if someone sends you and album from a wedding you were at you can pick out the photos you want.
3. Android users won't receive photos that with degraded resolution and color depth due to SMS compression.
4. Doesn't clutter up group messages as someone sends 15 photos into a group as you are trying to have a conversation. Often times causing people to not see messages.
5. The link sent can be accessed anywhere. So a PC user can easily download the pics at original quality without jumping through as many hoops. There are hoops but it relieves the most difficult ones that would require a wired connection from smartphone to PC for file transfer.
6. Speed of sending groups of photos could be incredibly slow depending on data connection.
7. The resulting battery hit with sending groups of photos/videos, especially when transcoding was required from HEIC and HEIV for compatibility mode has been lessened. This is essentially the export process with video editing btw.
The most important thing, the thing becoming a serious problem was the need for data conservation due to the resulting file sizes some photos can be with high quality phone cameras now a day.
This obviously saves Apples bandwidth since people can glance at photos without downloading the originals. And I'm sure was highly considered in the decision to implement this feature.
However the real issue were people sending albums (or albums size groups of photos) after an event. In a couple clicks (photos, album, select, select all, share, messages) I could not only send you all the pics from the wedding you were just at but I would use up YOUR entire data plan for the month and all of YOUR available storage in iCloud, completely on accident since the average user might not think about that.
Apples solution was to just send a well designed iCloud link that worked well in the messaging app and looked nice on a web browser. Turning the feature off isn't up to the sender, its up to the receiver (why do you care how I view my photos anyway? lol) and the difficulty with a toggle is the middle man (cellular network provider). Relaying that info could be finicky with iMessage and difficult in group iMessages and nearly impossible with SMS (currently at least).
Personally, as someone that has a limited data plan and limited iCloud space (I use it for my Macs file system) I appreciate the change. I was always annoyed when someone would just bombard me with pictures I didn't ask for especially when I was trying to do something work related. At least this way I can choose to look at them, choose which ones I want to use my data and also choose the time I download them (like when I'm on wifi).
TL;DR : You can't annoy, frustrate and potentially waste other peoples money as easily now....sorry....
