Mark,
Edit: I posted some comments that were utterly wrong, removed.
Well I have attempted to duplicate your change. At first I thought I was on to something, but it proved worthless.
I did notice one thing, your namespace is wrong I believe.
namespace XLite\Module\Alinc\AlincTheme\View\ItemsList\Upsel lingProducts;
Should just be:
namespace XLite\Module\Alinc\AlincTheme\View\ItemsList;
Still doesn't work though. I notice after rebuilding cache, that the new file we are creating is not copied into the var/run folder. Also since we are decorating the other class, I thought it would create a copy in the XC/Upselling/View/ItemsList directory, something like UpsellingProductsAbstract.php
Is it the same for you? No copied file in var\run\Module\Alinc\AlincTheme\View\ItemsList ?
It seems XC is ignoring our new file.