In the past most addons are updated free of charge to work with one (1) further version free of charge.
So addons created originally for FS2002 were generally updated to work with FS2004 at no charge to the customer.
Exceptions generally are those addons that either are superceded by new functionality in the new FS
version, or addons where the work needed to upgrade them would be too much for the developer to spend the effort without costing them massively (and thus potentially driving them into insolvency).
If a company with 2 developers on staff needs to employ these 2 people fulltime for 4 months to essentially rewrite the product that's hardly something the company can survive after all.
Older addons are usually dropped (though new products based on the same idea might replace them over time).
What will happen with this upcoming new version noone can yet tell. The product isn't even in beta yet, let alone anyone outside Microsoft having access to the information they would need to determine the upgrade effort needed and get started on the job.
The 737 will likely be made to work, Ultimate traffic I'm not sure (it's originally an FS2002 product and I believe the development team is inactive at the moment so there's noone to update it), I don't know what CS
are planning with the 707 but in the past they've updated some but not all their products, Aerosoft is usually good with updates, the others are too new to the game to have been through the scenario in the past.
Personally I WILL pay for new versions if I like the addon, especially as the updates usually include some new functionality.
But then I'm used to paying for upgrades like that, which are the norm in almost every other field.