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Technical correction: At least technically, "Metro" (assuming you mean the Start screen, charms etc.) cannot a replacement for the Explorer shell because it IS the Explorer shell. It is simply a set of new features added to explorer.exe, just like the taskbar in Win7. It doesn't run on top of, underneath, or alongside Explorer, it IS Explorer. You can see this yourself by noticing that killing the Explorer process also kills the Start screen etc. Metro style apps themselves can be thought of technically a new kind of "plugin" for Explorer.
Notice that the scenario you mention is only an issue for mixed use of both desktop and Metro style apps. If you're staying on the desktop, you've got the taskbar there as before. If you're just using Metro style apps, recent apps are available directly in the switcher.
I do agree that mixed use is kind of awkward, but if that's a problem for you it implies that you want to combine use of both to begin with. If that's the case, consider the alternatives that I've seen suggested:
* Making a completely separate OS for tablets etc. whose apps can't be used on other PC form factors at all. Obviously, if you want mixed use it's worse to be completely locked out of it than to have it be kind of awkward.
* Having a big modal switch that puts the whole PC into "desktop mode" or "Metro mode" (possibly when you plug in / unplug a keyboard or something). In this case, your scenario would involve first going into desktop mode, then selecting Chrome, then selecting a tab - so just as awkward as now. But it would also block a bunch of other ways of doing this and useful ways of using desktop and Metro style apps together. In your case, you also have the option of using alt-tab to switch directly to any app (Desktop or Metro style), going directly to Chrome if it's pinned on the start screen, using Win + <n> to go directly to it if it's the nth app pinned to the taskbar, etc. These wouldn't be available if you had to put the whole system into "desktop mode". There are other scenarios like using desktop and Metro style apps side-by-side snapped or on multimon, getting notifications from Metro style apps in desktop / desktop apps in Metro style apps, having a Metro style app / desktop app playing audio in the background while using a desktop app / Metro style app, etc., that would be pointlessly blocked by this approach. The only advantage is that it's conceptually simpler, but other than that it would make everything more awkward and nothing less awkward.
That's not to say that no better approaches are possible here. I agree that "Microsoft could integrate the two much more efficiently and more fluently if they wanted to" but I disagree with "easily". It's tricky to design something that both integrates them fluently and still keeps them distinct and maintains the identity and benefits of each model. The "obvious" solutions like integrating the taskbar all have their own problems if you think about them. Considering this I think Win8 is OK for a first effort in this regard, hope to see improvements in the future.