An important point that people seem to be forgetting is that many plugins, including regex most likely will be included in the program to start with, so cross plugin dependencies aren't so much of an issue. Granted, you're right that more files mean a slower load, but for less powerful systems (not everyone is using a new-ish comp), the size of the binary does matter. I really don't mind taking other plugins and distributing them and keeping them updated, but I think it'd be bad to include every sensible plugin in the source.
Problem of dependency is not "I hope the feature I need is there", it's about:
1) How to summon a feature you don't know is in which code from other code that can't reasonably know the internals of the target code.
2) Do it efficiently.
3) What to do if feature isn't there.
Libraries as regex, if properly embedded, can grow an exe file of 70-100 kb. That size isn't much of a problem to extra load on ANY machine, including modern cellular phones, but if you really want you can try to see the difference in load time on a x286. Is that old enough?
Well, on that machine, if my memory isn't wrong, disk transfer time was about 1MB/sec which brings load time of our regex to about .1 seconds. Let's say we fix size of extra features for the engine in 1MB. You'll have a lot of extra features in that. Probably more than we could ever stuff in. But let's fix it to 1MB. You'd have about 1s extra load time; let's set another second to apply the global relocation table (I guess it was less on my 286, but let's stick to it). It's 1, 2, and it's there. Or in the case of a poor 286, it's 1..10 for the program and 1, 2 to load the extra stuff we want in.
Do you really want to bog down runtime performances and start get hairy code by not providing basic functionalities everyone wants for THAT?
The more services the engine offers to the plugins, the more powerful, simpler and efficient will be the plugins.
I.e. what if Mozilla didn't offered XUL? -- you wouldn't have chatzilla. And XUL decoding is a damned big piece of code...