[jacktheking]

How feasible is it for the whole website to be flash based?

There is a lot of disadvantages to have a Flash-only website today, including search engine optimization and software-dependent. With the release of powerful HTML5 and good client-based scripting language, JavaScript, will both of them be able to take over flash in the "interactive-web' development industry?

(Flash uses ActionScript).

[earthsalt]

A website that is flash-based is likely to give you problems in relation to various performance parameters. A website needs to be dynamic enough to align with web components well.

[Sam]

Two big issues for Flash:

Device Support (from Wikipedia):

The latest version of the Adobe Flash Player runs on Microsoft Windows, Apple OS X, RIM, QNX and Google TV.

i.e. no Android and IOS support for flash.

Open Source...
ActionScript is not. :/

ActionScriot is somewhere between the 50-100th most popular (depending on the Tiobe index definition of that) programming language in the world as of February 2017. Javascript is 7th. http://www.tiobe.com/tiobe-index/. Javascript has a big advantages over ActionScript. You can use it server-side with NodeJs. Javascript is completely cross browser supported. Languages compile to Javascript (e.g. I was exploring learning CoffeeScript because of it's Pythonic nature but sadly the updates aren't there. There are other languages which compile to JS which are beneficial depending on the programming background you come from).

I'm sure having a Flash / ActionScript website is feasible... (I'm not sure how you would insert into/ retrieve from a database) but under what circumstances would you? People pick frameworks and languages from their background e.g. Django for Python or PlayFramework for Java OR because they're new to programming and want to learn something that's widely supported e.g. PHP. Even using other languages with ActionScript / Flash doesn't seem appealing when there's so much support for the big languages.

[thisnthat]

I think you answered your own question. Another disadvantage of Flash, as noted by Sam, is it's not going to work well on all devices. The number of people visiting sites by phone these days is huge. Why leave all those people out of your audience?