You can remove the DOMSubtreeModified event listener first, then you are free to modify "locked" content as desired. I hope to not see a day where the Web Inspector lacks proper facilities to inspect and modify a page.
I recall when early developers would make it so you couldn't right click and view the source. You'd get some silly alert() dialog saying hey your not allowed. You even had users who would attempt to "obscure" the html by converting it to a encoded string that anyone could easily decode because it had to be for your average browsing user to actually see the page.
If this trend were to come back I would hope to see it quickly fade away because it'll just become a game of cat and mouse in which ultimately if you want your average user to see the content you have to give them the keys. Hopefully this type of locking becomes a thing of the past like it has before.
Unfortunately, since the W3C caved on EME and DRMs apis for videos (i.e. plugins), there is a possibility of seeing that scheme being extended to images or text content. They caved once, there is absolutely no guarantee they wont cave again in the future and put the interests of a few companies before the idea of a free web.
That is worrisome but as always it's a standard thus not all browsers will implement on top of most browsers now have some way of configuring deeper settings like this to be disabled.
In the end the data goes to the user so a way would be found with or without Chrome or any of the other big wigs. What they should do is focus on how to brand it better so users know where it comes from - this would create a way to spread free idea's while giving credit where due and hopefully pushing the idea of the free web further.
You can also block the script. Really - most people who'd know to modify the content via the DOM inspector tools can probably figure out trivial ways around the script.
If this trend were to come back I would hope to see it quickly fade away because it'll just become a game of cat and mouse in which ultimately if you want your average user to see the content you have to give them the keys. Hopefully this type of locking becomes a thing of the past like it has before.