The problem is that "your chosen extensions" can cause worse bloat than an unused feature in a browser. I'd rather have as much functionality as I can from the developer of the browser itself. Extensions are helpful (particularly for obscure features that no browser developer would bother writing because the user base would be too small) but all to often they break more than they fix.
Basically, the Vivaldi browser is designed to appeal to people who miss Opera 12.x. When Opera moved to Chromium in version 15, it did basically what you are talking about--stripped out nearly every feature aside from browsing itself and it opened up to Chrome extensions. But, many of us found that, in order to add extensions to Opera 15 and later, that met the features we used from 12.x, the browser was a hulking mess--and the extensions for the most part don't work as well as the built in functionality from 12.x. And the whole thing was now slower and riddled with memory leaks due to the extensions.
So, basically, I'm not going to suggest that you must switch to Vivaldi, but personally, I am keeping my eye on the project. I think there is a good user base to be had out there for it.