What did you need plugins for? Netscape and MSIE both were able to add a script tag to the DOM dynamically, and that was all you needed to build the rest of it.
You were able to use document.write somewhat dynamically, but the not reliably across all the browsers. Even if you did make an RPC call, there wasn't a lot you could do with it presentation wise because DHTML was two years away. (Even then, IE was the only one who had a decent implementation for a number of years after that.)
Plugins and applets filled a huge gap functionality wise.
My recollection at the time was IE4 had a working DOM while Netscape's DOM was buggy and left a lot to be desired. It was there, but it was a struggle.
I don't say that to crap on Netscape. Microsoft was a well-funded company with a lot of experts who had years of experience create applications with DOMs. Also, they had the luxury of delivering a browser for one operation system.
Netscape was a brand new company that went public 7 months after it's first release and 1.5 years after its founding. They sold investors the premise they were going to become the Microsoft of the internet. They had a fraction of the resources while they were trying to creating browsers, email clients, collaboration software, web server, and a host of other software for multiple platforms.
Plugins and applets filled a huge gap functionality wise.