I would guess that the answer to your question may be different depending on where you are in the world.
In the USA I think, rather than declining in availability, wet shaving supplies have expanded to meet the increased demand.
I see parallels in this to the expansion of the craft beer industry. The demand for unique brews has resulted in many new beers that were rare or unavailable in years past.
In terms of wet shaving products, I think the market will continue to expand, and products will likewise become more available.
However, because the shave boutique shop around the corner may disappear, some will predict a decline in the availability of shaving products.
I believe that this is more related to a distribution change rather than an actual decline in the market.
It is hard for a "brick and mortar" store to compete, especially if they don't have a web presence.
Relying on walk-in business limits your customer base. And the increasing diversity of products puts stress in the local retailer.
They just can't afford to stock everything shave related.
Products that remain popular will continue to be available either locally, or via the internet.
Prices, however, are a different factor, and some things may become more expensive - especially if you factor in the cost of shipping.
DON'T PANIC!
Shaving will continue to be a popular activity - so long as facial hair (or other body hair) continues to need regular attention.
YMMV

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