Because it is not “universal throughout all applications.”
There’s been a change in philosophy recently where developers are asking: If what we mostly want when scaling is to preserve proportions, why do we need to hold down a modifier to do what we want most of the time? This reasoning has caused many developers to swap the default so that proportional scaling is the default. They’re basically saying, do we need to maintain a precedent when it’s probably not the right thing to do? Should the way it was done in 1990 be unchangeable for any reason?
For example, let’s suppose someone gets so angry about this change that they decide to leave Photoshop and put their financial support behind a competitor instead. They delete Photoshop and pay just $50 for Affinity Photo, one of the most powerful and widely praised Photoshop competitors. The second they decide to scale something by dragging, they will find out that…Affinity Photo scales proportionally by default too! Pressing Shift scales non-proportionally. Just like Photoshop does now.
In exasperation, let’s suppose they try Pixelmator, another popular Photoshop competitor. Guess what, it scales proportionally by default too. Those are just two of the examples. These days, the applications that require the Shift key are mostly older ones such as GIMP.
Affinity Photo and Pixelmator are some of the most modern code bases out there, with the most current UI thinking. They represent where photo editing software design is going, when the big picture is considered.
Part of the big picture is that not using the Shift key is also more consistent for applications that have versions that need to work on touchscreens, because modifiers are more of a hassle on touchscreens, especially when no keyboard is attached. If no Shift key is required to scale proportionally, the same uniform gesture works across all platforms. This wasn’t an issue in 1992, 2005, 2010…but today, with Photoshop on Surface tablets and on the iPad…
I’m not saying the situation is perfect. It’s actually a big hassle if you use multiple Adobe applications, because the others still use the old way of scaling. You can easily get mixed up when switching between Photoshop, InDesign, Illustrator, etc. I guess that’s part of the reason they provide a preference that lets you turn off the new behavior in Photoshop.
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