This is probably TMI, Eric. My advice is to do what is comfortable for you and your goals at this time. Remember HP is a community of writers that is diverse writing within a wide array of topics / Niches. Anyway following is TMI . . .
I don't understand "popping up and asking for submission of my stuff to the promotional stuff". Are you referring to the queue at a Hub to 'Submit to HubPages Network Site"?
A footnote all your articles have ads on them, so they advertise. (Unless you disabled ads or HP did because of breaking the Terms of Service someway) All your articles recommend other Hubs, so they promote stuff.
A Niche Site is a promotional site in the sense it is a focused WebSite promoting like articles and topics. It is not more advertising. The only advertising done is on your article. The specialized Niche website does promote other articles at their homepages.
No the New business model does not have to be embraced. One does not have to submit any of their articles. There aren't enough Niche Network Sites to cover all the topics to begin with. They do cover the most popular and successful areas.
A personal note is my 'life' niches doesn't fit a viable niche to write on. There are many reasons, but I still write and publish.
There are times an article is written will have no Niche Site to go to. Owlcation has a Topic area Humanities where there is theology and philosophy. I dun'no if spirituality would fit . . . Here is a link to it:
There are advantages to an article moved to one. In the perspective of getting more traffic that equates to more business success. They may be chosen by the editorial staff if it fits on one those 25 Niches, meets quality standards, and has some kind of 'steady' traffic flow. And, we have the option to submit. It will be reviewed for quality and etc.
Here is a link to the Editorial Policy for Niche Sites: