In these cases they've just used concrete5's SEO tools and features, and then for changes that have required actual template or code changes they've simply referred back to us and we've spent the hour or so to implement the improvements.
We've not had any complaints or suggestions that Wordpress is needed - I personally wouldn't worry about that as an issue, or at least don't worry about it until it actually happens.
If that did actually happen, I'd directly tell my client that a good SEO company can work with any decent CMS , and that concrete5 is perfectly suitable for SEO work. I'd actually go as far as telling my client that if their SEO company suggests rebuilding their concrete5 site in Wordpress just for SEO reasons they don't actually have a clue what they are doing!
True on-page SEO isn't about plugins - it's about page titles, slugs, headings and adjusting text content, and concrete5 handles all that. Plus it's got the tools to carefully adjust search engine titles and descriptions easily.
One way to approach it is to ask them to get a report or list of SEO recommendations from the SEO company, rather than letting them loose on sites.
SEO needs to be part of a holistic process and bolting it on afterwards is a fundamentally flawed concept. Frequently, the job is sold as an additional service for a live web site by an 'expert', then handed off to a junior who repeats the SEO company 'formula' without any understanding of the real business requirement.
So for good SEO, get real expertise on board and design it in from the start of a project.
We have seen great results.
Although I am based in the UK. I have been working with an SEO partner for the past few years. We have sites in Oz and USA as well as other countries.
I would be more than happy to work with you to get your clients the best results possible.
I do my utmost to make SEO-ready sites up front (including all the tasks mentioned above, plus sitemap submissions, Google My Business registration, Google Search Console registration, 301 redirects from current pages, analytics, etc). The main issue is ongoing tweaking: monitoring keyword performance, SERP, etc. Quotes for such work often exceed the client's income!
I'll pass on relevant details from here to my clients for their consideration. I'll also make the offer that I'll talk to any SEO guru they choose, to familiarise them with the 'concrete5 way'.
Be sure you have the right person to enter your SEO though. As they all claim to be "the best" and ask "low prices" and such. Thing is, it has to work for your site and they have to prove it in some way. Me personally dislike services like paying a monthly fee and them having their own "CMS" kind of thing next to the existing site. So look up whatever you can if you find someone to do SEO!
- I don't do SEO for clients either by the way. You just need some local contacts to actually get in touch with, instead of being ripped off by others haha.
All that buggy concrete5 beta sh^t is of no use.