If you have a specific product in mind, I don't recommend the strategy. Find investor funds and build the product.
If you don't have a specific product that you want to build, then I think it's a more viable path. Running the agency gives you a stable revenue and base to bootstrap a product when a product opportunity becomes available. Running an agency also gives you insight into pain points that multiple different companies are hitting.
So, an agency as a path to eventually developing a product makes some amount of sense but an agency as a path to develop a specific product vision probably doesn't.
FWIW - I've also seen a lot of SAAS companies come out of individual consulting engagements. I know of two services that started with "I'll give you a 25% discount on my normal rate if you're ok with me keeping the IP for this portion of the project" type negotiations.
I think this is a neat way to start as it's validation (someone is paying) and your first customer (which is the hardest to get) all in one shot.
Also puts a lot of back-office infrastructure in place to serve as a starting point for a transition rather than have the founder(s) have to manage that as well.
If you don't have a specific product that you want to build, then I think it's a more viable path. Running the agency gives you a stable revenue and base to bootstrap a product when a product opportunity becomes available. Running an agency also gives you insight into pain points that multiple different companies are hitting.
So, an agency as a path to eventually developing a product makes some amount of sense but an agency as a path to develop a specific product vision probably doesn't.