I’ll leave this here for now, this is a topic that believe it or not has been discussed to death several times over the years, either here or in IRC (I think more often in IRC).
There are several issues with including a ‘basic set of plugins’.
First and foremost is developer time. The majority of development of Ardour comes from two very specific people. Those people have focused on developing Ardour itself, in part because third party plugins exist, and can fill the hole and need.
The fact or not of it being difficult to install the third party plugins is really outside the scope of Ardour, as it depends on the platform, the packaging and the distribution of the plugins themselves. Ie. On Mac and Windows you have some plugins that are easy to install, because they package themselves in nice installers that take care of all the work for you, but you also have some plugins that users struggle to install because you need to manually place them in the correct locations, and for instance many newcomers especially have no idea that /Library/Audio/Plug-Ins exists on Mac (Going off memory for the location, may not be correct). The same is true on Linux really, the difference is on Linux plugin developers depend on the package distribution system of the platform to distribute the plugins rather than package it themselves, for lots of reasons outside the scope of this comment, but they could package it themselves as well and in fact there are a few (Particularly commercial) that do.
Second thing is developer skill set. Despite what I said above, Ardour is NOT developed by JUST two developers. Even so, there is a large difference between developers that can submit code to Ardour, and those that can write and maintain proper DSP code. So amongst the tiny fraction of people that use Ardour, is an even tinier fraction of people that can write code and help with it’s development, and amongst that is an even tinier fraction of people that can code audio DSP correctly.
Third thing is maintenance. Even if Ardour was to include a set of plugins in its distribution, first who picks what plugins are acceptable for inclusion? This could be a large list of potential plugins, each person has a different ‘need’ and ‘idea’ of what should be included. Should a chorus be included? Maybe, but what about Flanger or Phaser? What about more specialized, dynamic EQs, noise reduction, etc.? And maybe multiple EQs, compressors, etc. The answer will vary depending on who you ask. So are all of them included? Even if that were possible, the list would grow long and become overwhelming for the first time user to find something that does what they truly want. So who picks this?
Even beyond who picks the list, who maintains it? For instance years back, there were people commenting that the Calf plugins should just be included in Ardour (This is 10+ years ago at least). I was even one of them (Again remember this was a LONG time ago and Calf was being actively maintained), as I teach introductory audio mixing at a local uni, and want to make sure my students have a good set of tools to begin mixing with and explore the basics. Ok… so what happens when we hit a situation like we have in the past decade or so, Calf is no longer maintained, and even due to how it is programmed is just flat out not working. Now we are back to our developer time issue above.
Part of the reason ACE plugins exist right now is because a specific person has agreed to maintain them (Not one of the two primary developers by the way). That person still has limited time available, but has maintained the plugin suite specifically for Ardour to avoid the issues before, and had a proven track record of maintaining plugins like this long before they agreed to do it for Ardour. This isn’t something that someone can jump out of the woodwork to agree to do because the last thing anyone would want is ‘This plugin included with Ardour crashes my session’ and if someone tries to maintain them that doesn’t have the skill set, or the commitment, that all falls back on the developers of Ardour again.
Now I will not disagree, it is nice to have the ace plugins installed by default. But considering I teach introductory mixing, this is a problem I run into a lot, and honestly I collect a collection of plugins and tell my students to download and install them instead. This solves the issue in the way it was intended, by allowing third parties to come in and fill the hole, it also teaches my students about plugin management and installation, so that they can find additional plugins on their own and install them as well. And believe it or not, this seems to work well enough,
Do I think the ACE plugin suite COULD be expanded? Yes. Do I have the time, knowledge and skill set to do so myself? Nope not at all, I can code but I cannot write DSP and in fact really don’t understand the math behind it much as I never took calculus and most modern DSP seems to depend on calculus (It can be done without calculus, but that seems to be the rarity). Which means I am grateful to the people that do develop plugins, particularly open source plugins, for the time and energy they put into it. I have considered using some toolkits that make it easier to build a basic set of tools (HISE for instance) but I have not had time to develop that. So again, grateful to those plugin developers that choose to do so, and particularly to those that release the code for the plugin to the world for others to use and help maintain as well, though as we have already seen with Calf, this is not a guarantee that the plugin will in fact be maintained.
And lol this doesn’t even touch on the quality of the DSP code in Calf for the record which is why even then I gave up on packaging it because it was obvious that wouldn’t be improved. 
Seablade
EDIT: Important Spelling (See comments below)