Hi Rebecca,
It’s possible that the big (underlying) objection causing your prospect to be adamant about using WordPress is that many businesses want to manage their own content after handover - once their site is tested and fully operational.
Due to its longevity and the amount of info and free training out there WordPress has become “famous” for its usability for even a novice. I disagree with this as it can be a total minefield for non-techy types and we all know how badly that goes.
I’d cover this objection in a video - as also suggested in one response here.
Getting inside the head of your “ideal client” and covering all of their pain points and objections before they even get to talk to you means you can have a much more powerful conversation with them when you do connect.
I have a motto - never say NO to a client if they want something done a certain way when I know I have a better solution. Say YES first. That removes the friction. Then in the space that opens up you could say (for example in this particular case) - "Wordpress is great because (keep this simple). Why do YOU like WordPress so much? (listen intently to their reply, and go deeper with this questioning so they reveal more underlying beliefs with each new “why” from you).
This short conversation will give you the fodder for how you will be abe to address all of their “WordPress desires” by using Webflow “but with all of these EXTRA benefits”. It should be a “no brainer” for them to go with Webflow if you have not ONLY covered ALL of their WP desires but added a bunch of extra benefits to them and their business by using Webflow instead of WordPress.
Once again I remind you that this conversation will have the most powerful outcome if you know your “ideal client’s” pain points. Therefore doing your homework on your ideal client and building all of your marketing around addressing THAT client’s pain in your marketing will attract warmer leads for you to convert into sales (with your new approach to the sales conversation 
FOOTNOTE
I’ve been in sales and marketing my whole career. Have had several businesses over 40 years. My last (until 2015) was as an agency owner for 19 years. So I sit very much at the intersection of SALES /MARKETING / PRODUCTION / CREATIVITY as the Catalyst that makes things happen and manages projects end to end.
I’m currently developing my next biz - an online “school” for small business owners - courses, training - and coaching (am accredited as a coach) across design, marketing and income growth. Just sayin’…I rarely have time at the moment to comment on posts but I decided to “enjoy my morning coffee with you today”. I wish you every success. Go get 'em Goddess!