In an age where it seems nearly every day we get invitations in our mailboxes to submit or review for a new journal, or yet another conference we can attend- is there maximum capacity before we begin to dilute our professions?
While I am not sure about the maximum capacity, we already have, at least in the sciences and in maths, three groups of journals,
a relatively short list of journals considered by pretty much everyone to be good
a larger set of journals in the grey area (decent or at least not bad)
a yet larger set of journals which are problematic in one way or another (e.g. the journals from the Beall's list (http://scholarlyoa.com )
and I expect the strengthening of separation among these groups, especially the first two and the third one. Even now what we usually see is that the publications in the journals from the third group do not carry much weight for situations like promotion or tenure, and this trend will likely strengthen in the future.
Something similar will also probably happen for the conferences.
It may also happen that the scientific community will develop some sort of ranking system for the articles no matter where they are published and this ranking system will serve as a kind of an umbrella journal (or a megajournal, if you like).
I'd say that there is certainly room for new journals to improve things. In some sub-fields, having more (or more specialized) journals can be very useful. In other cases, new journals are doing things differently in terms of open access, refereeing, etc.... It's also important to have journals which are more open in what they accept, since so much work is incremental and you don't want to limit publication to only the most high-impact works.
However, just adding more journals can dilute things or actively worsen the situation, in particular in the case of very low quality or otherwise 'problematic' journals, as Artur notes. Unfortunately, it looks like a large fraction of the new journals fall into these categories, especially the ones that are very active in sending invitations to submit papers. There are, of course, perfectly respectable journals which solicit contributions, so this dramatic expansion in the number of journals makes it harder for an early-career scientist or someone moving into a different field to know where they should be looking to publish.