this is not an effective long term strategy in a collaborative environment that is advancing for the same reason that having a private secret fork of the linux kernel with a few proprietary improvements is not an effective strategy.

integrating your own work with the latest public advances takes resources. For one or two small changes this is manageable, but the further you diverge from the public, the cost of maintenance rises exponentially if you want to continue to integrate public advances. when you publish your meaningful advance, you offload the maintenance burden onto everyone else (and they only have to pay a linear cost rather than an exponential one) as it's integrated by default in new work.

In most cases, the (exponential) maintenance cost of integrating public advances with secret ones exceeds the value of the public advances, so most that undertake this strategy of advancing the open frontier in secret don't attempt to integrate continually, but instead try to make a breakaway sprint in isolation to grab a few sticky customers before the unstoppable wave of the public frontier catches up.

This is a pattern commonly seen in university research departments when researchers switch into product development mode, most of these projects are a sprint to advance away from the public frontier once a good idea is found and they do good work and find a few customers for a little while. But if you check back in a few years you won't find an advanced research department but a zombie IP company that brings in a steady income via IP enforcement and a small number of customers for whom switching is too expensive.