Most Manufacturers include some degree of resource management in their planning efforts. For example, Ducati has been known to invest significant effort in their marketing/racing initiatives long before the actual model year starts.
Unique models are at a disadvantage because even if they exceed forecast sales demand, the financials don't always support manufacturing accessories if they are sold out. Conversely, if a Bonneville's particular accessories for that model year are sold out there is a greater likelihood that Triumph-Manufactured alternatives will eventually be available either for that particular model year or a different model year that will still work.
My former 1999 Adventurer was succeeded by two additional model years, but Triumph cut way back on sidebags and windscreens. When the last model year came out, there were almost no accessories available. For a while, almost any sidebag on eBay was sold quickly - regardless of condition. Dealers and Shops that had the capability would source the worn parts out for restoration and resell them on eBay. I watched from the sidelines because my bags were installed when I bought the bike.
On my former 1995 Daytona900, general carb/airbox parts were still available from Triumph. However, when the mechanical issues became too numerous I was considering a complete rebuild/restoration and was able to build a budget from parts and body items available on BikeBandit.
The new Triumph Thunderbird shares some parts, but is a completely different motorcycle. I think it's being well-received, but nonetheless accessories are being managed in a similar way to ensure that no one is left holding inventory.
This is all just my opinion, from years of involvement with Triumph.