Hi @arunsathiya, I think it's a complicated question, but I'll try my best to answer:
Earlier Google 'officially' said that they prefer URLs with unique IDs so that it'll be helpful for them to analyze or something and it's easy for Google News as well. So this was one of the main reasons why most of the old WordPress blogs preferred %post_id% thing. But later Google removed this from their official doc, and they are okay with 'News Sitemap' now.
Apart from this, according to me, I wouldn't prefer example.com/hello-word format for 'posts', because this is how a page should look for me. Again, I wouldn't prefer example.com/category/hello-word format for 'posts', because I keep on changing the categories like 'news', 'how-to', blah blah.. my mind is a monkey.
We all know we hate when we see outdated articles, take this article for example, even though they might have covered all the latest tips, when they see '2014' in the 'URL', it'll be kinda hard for us to click. Possible right?
So, finally and finally, the only possible format I can think of is:
I'm sure the 'post id' in the URL will be as long as '3245345344' after a few years, but I don't think that'll bother me, most of the forums have this 'unique id' in their URLs, take Discourse or Stack Exchange for example. This might also help the server to properly assign each and every 'unique' thread.
I'm still researching for a proper reason, I'll definitely update that here soon.