Sofromlex:
I would guess from the many comments I've read, the consensus is, the best alternative to iWeb is EverWeb. I agree, but before I came to that conclusion I personally tried Goldfish, RapidWeaver, Sandvox, and an offering from my hosting company [MacHighway] called Weebly. I finally ended up purchasing the inexpensive EverWeb. If you haven't built up a huge inventory of iWeb pages [as I have, over the years] go directly to EverWeb. I'll explain why further below.
The advice given in the Apple Store is at best uninformed. Somehow iWeb has survived every OS X update and functions just as well as ever, except that during the last update [El Capitan] it developed an easily-overcome little bug, that most of this discussion thread has been following. But it's a reminder that if having the latest OS is really important the next update may no longer accommodate iWeb. It's a calculated risk. I for one keep one of my iMac's on Mavericks just to play it safe.
EverWeb was designed specifically to look and feel like iWeb. It is still being improved yet already has so many features that one wishes iWeb always had, such as Master Pages, ALT Text, Header/Footer Code, settings for Mobile devices, on and on and on. To me the best thing about the program, as with iWeb, it allows one to start as a blank page without using those dippy-looking templates, and completely customize every aspect of the website design. [They also provide dippy-looking templates, if you secretly prefer.] There are still some shortcomings that a fussy designer like myself has shared with the developer. But there again is the good news, the developer is very responsive and it appears that this product will be supported and maintained for a long time. [No, I don't have an interest in the company; my only interest is to be able to continue making websites that meet my expectations.]
I hope that helps...
Richard