Morphic is vital if you want to make a lively language, i.e., one in which you can drag a block into (or otherwise edit) a script while it's running, and the change takes effect immediately. Morphic gives very detailed control over things like which object sees the drop when you drag a block onto a pile of overlapping objects and let go. Early on, in the development of Snap!, in certain situations it was hard to drop an input onto the input slot you meant, and Jens fixed it by giving priority to empty slots over slots that already had something in them. You have to have your own widget manager, Morphic or equivalent, to be able to control the user interface at that level of detail.
But that's not how most people do it, because the browser has its own window manager, called the DOM, and if you want to put things in a window it's much easier to use the DOM than to roll your own. Besides the obvious point that if you use the DOM you don't have to invent your own window management, the browser makes a lot of things easy that people don't even think about as issues. For example, the DOM provides text copy-paste, specifically taking charge of which object you're pasting into when a bunch of them overlap. For the longest time, you couldn't paste into BYOB text fields, and specifically, you couldn't copy your password from your password manager program and paste it into the BYOB login window. (By the time Snap! came along, that problem was solved.) With Morphic, as far as the DOM knows, the entire Snap! window is one big bitmap canvas, so it can't help you organize text fields.
When Jens rewrote BYOB as Snap!, we had a one year deadline. Jens spent the first eight months of that year implementing and debugging the Javascript-based version of Morphic. I was terrified! But once Morphic worked, he wrote the whole rest of Snap! in four months.
There are other costs, too. Browsers come with huge libraries of user-contributed extensions. I have 14 of them installed in my Chrome and even more in my Firefox. Here are a few of them:
Dark Reader, which provides dark background viewing on a per-site basis (because some sites break if you do it the simple way).
Duck Duck Go Privacy Essentials, which removes advertising and tracking from the pages you view.
LastPass, a password manager that remembers all your passwords for you (because that makes it easy to use a different password on each site, which is the right way to protect yourself).
NoScript, which removes things from web pages, like the Duck Duck Go one, but with much more detailed control over different types of threats. (The cost is that you spend your first few weeks with NoScript tuning it up for your particular level of paranoia and your particular exceptions, whereas DDG Privacy just runs out of the box.)
Volume Master, which has per-site volume controls (so you can crank Snap! projects way down, for example).
... and so on. Anyway, the point is, those extensions modify the processing of particular kinds of DOM elements, so they automatically work on web pages implemented in the DOM, but they don't automatically work on Snap!. Having to implement our own copy-paste is an example; that would have just worked if we'd used the DOM.
