There is definitely a market, but I'd say it's a difficult nut to crack. You first want to do a quick cost-benefit-analysis first to make sure you're making an informed decision.
We won't be factoring privacy, working with a big company, or other concerns in this thought. We will only focus on the numbers.
Lets look at email/productivity market real quick.
Google's base package is 6 dollars a month per user with all the backups, support, and infrastructure in place (realistically, you'll want the 12 dollars a month package).
Since email is critical infrastructure (I believe it's one of the most critical elements of a business), lets say two DigitalOcean VPSes at 6 dollars a month each (one primary, one failover) with a license of HostinaBox/Vesta/Whatever Open Source Solution you use with DigitalOcean's backups enabled. Not the "best" design but something I'd say is OK and within scope for a small business. That's 12 dollars a month base costs + your consulting time for a sizeable capacity.
For a small business of 1 person, they get a better deal by just going with Google. Google has their applications and softwares easily integrated with their other services, comes with their productivity suite (GSuite == Google Docs, Drive, etc.), and as a small business who is probably risk adverse in their decisions when it comes to these things, feel more comfortable working with Google.
For a small business of 5 people, I'd say it's still more worth it for them to use Google as that's 30 dollars a month (most consultants charge more than that an hour). If they hire a consultant and if the poo hits the fan, then they'll be paying a consultant money to execute the disaster recovery plan. Even if you did take them on as a client, that's a maximum of 18 dollars a month you get to keep (assuming no issues/errors happen).
For a small business of 50 people, then now it gets to an interesting territory. However, for 50 people I'd change up the base server/system configuration to have higher capacity, more fault tolerant, and resilient under disaster scenarios (which would increase base operating costs).
I'd say this really depends on marginal benefits and based really on relationships established with your clients. In the end, you can probably make some $$$ but your time and effort might be spent on more productive and lucritive tasks. This is also assuming that the self-hosted OSS software is of quality that the clients will be happy with. I'd argue Google's mail offering may have annoying/restrictive spam policies and be frustrating at times, but they have a high quality product made at an affordable price point. The variation in quality of OSS products concerns me as well as developers who are probably overworked and underpaid for their contributions asked to make changes to support clients they're not directly paid by.
As a risk averse business, I'd rather rest my eggs in the Google/Microsoft/whoever basket and directly work with the "entity" that maintains the codebase (or has the talent/expertise on-hand to make adjustments) rather than a middle intermediary of equal level but, in the end, is subject to the decisions and leadership of the OSS product.
Now take this to another step and say you build a consulting company that handles all of these as a one-stop-shop? Well... Then I don't see anything new service/model here than what a local IT consultant company can offer.
So to really make this work I'd say a shop that automates these deployments on-demand and offers a large selection of applications to use is probably the best step forward. Even then, I don't really see the viability of this on a funding perspective except for scaling. In the end in my perspective, the opportunity is there but it'd be hard to do it right. Also funding the developers of the software you're making money off of would be great, but that's a whole nother thing (and I can squabble about that for hours).
Quick plug: two very good friends of mine are in the process of tackling a similar issue via their venture[0]. I am a customer of theirs but have been friends with them even before this venture. Really recommend their product as an affordable and reliable product that "just works".
[0]: https://mxroute.com/