As for Photos, how many plugins will one need for an equivalent experience? And how soon will you wish you were back in 2005 when Apple quite rightly saw that people were fed up of a string of programs and wanted their workflow to be in a single pro-oriented program?
The number of plugins required would depend upon individual needs of what is desired above and beyond the actual capabilities that the new Photos app will offer on it's own. Currently, even the Apple engineers working on the OS X Photos app quite likely don't know what v1 will actually offer ... it's early days. It could be no plugins at all for at least a few users ... to a rather lengthy list for others ... Just as it is for Aperture and Lightroom users today. Some use no external plugins ... others have a lot they rely on for their workflow.
If, for example, Nik or onOne could offer their suites as part of the RAW pipeline with the Photos app as the baseline for browsing, culling, metadata and basic image processing ... that would be all many users would likely require. I know I would be much more inclined to invest in plugins if they would fit into the workflow without resorting to creating derivative files to accommodate the use of plugins.
Yes, in 2005, Apple "rightly so" offered up a singular option ... their ceasing further development of iPhoto and Aperture may not be a reversal of that intent. That concept is over a decade old now (since the time development began) they could very well be onto a newer, more advanced concept that is even more "rightly so" for this point in time.
We do not know all the facts yet as to what this new offering will be capable or exactly how third party extensibility will all fit together ... I have a feeling that it won't be quite like "plugins" as we are accustomed to ... but a furtherance of a more seamless use where the user experiences little difference between where the parent app ends and the supported extensions begins.
Which is exactly what I had hoped at one point was Adobe's intention for Lightroom. If they had chosen to focus on solely providing Library, Develop and Print package of modules ... then open up additional module development to other developers ... it would be an awesome foundation while the lesser modules could be addressed for the various niche markets like slideshow, books and web ... or even others. In that respect, I think Adobe may have missed the boat.
I think it is a bit early to pass judgement on how it all fit together until we see a bit more of what Apple has envisioned.