Hi There
It's definitely hard to say, without intimate knowledge of all aspects of the site - what the benefits will be by moving to WordPress. As a standard answer, I always see it as beneficial to move to WordPress. But the client should understand it's an investment, it's not like they're going to instantly rank better just by moving to WordPress. What it's going to do is put them in a better position (in a lot of ways) to rank better moving forward.
So in general, I'd always say "worth it" to make the move - but it's all in the specifics in the current site and how well things as executed.
It is entirely possible to have minimal link loss through redirects. You just have to;
- Avoid chains
- Make the redirects on a page by page level - and the topic needs to match (as Cyrus does a great job explaining here.)
You can fulfill both #1 and #2 no matter what platform you switch to, no matter how old the site is. You can always go back and undo redirect chains using the methods in the post I shared.
I'm not recommending a new design - I'm saying the process of how you go about implementing on the redirects can be a little more straightforward (and thus eliminating potential for chains and link losses). You first build the site in WordPress, without worrying about URL structure, .html slashes and so forth. Then you redirect old pages to the new pages however the URL structure ends up. You can go to past redirects and redirect them directly to the new wordpress URLs.
Hope that all makes sense - summary being - it is worthwhile to switch to WordPress - and any link juice loss is the fault of poor implementation, not the result of the switch itself per se