I think that in the longer term, if they dont make topics easier to navigate and sort info from - they will lose the value they provide. For example - you cant easily browse the topics, top answers top questions etc.
The search box tries to be too cool and provide a list of what you may be searching for - but at the expense of letting you type fluidly.
The choice of fonts, sizes, colors in the UI is counter-intuitive and suggestions made to the quora team fall on arrogant ears "We designed it to be like that" rather than, you know what - you may be right - useability is affected by the slowness of the site"
Any thread with a substantial number of comments regularly crashes my browser (the thread about which startups are hiring in SF Bay will crash FF on linux) - and viewing the site on a phone is clunky, jerky and slow.
All of that aside - if they can allow people to post as quickly and efficiently as Reddit - and allow for topics to be better navigated, I think they will survive.