No one has mentioned penguin,AMP and HTTPS !
These are the main issues you see. Google has a new (well ok i think its 18 months - 2 years old) algorythm out, alot of older site got hit by it. Older school SEO actually goes against you in the search algorythm, so many of the older sites are run by people who paid for SEO when the site was built, it is now 'wrong' kind of SEO.
Then you get AMP, Accelerated Mobile Pages. This is really nasty, we have responsive designs and those work well, already a big enough pain to design a responsive site!! But AMP is a google product, i am REALLY anti this, its no business of a search engine how i present a site. I refuse point blank to conform to the way a search engine wants me to layout a site! They should fit the site not the other way around.
The difference between being responsive, and conforming to the AMP standard is all google favored. Its mainly to make indexing much easier and cheaper for them, its also much quicker for them to index if everyone uses the same 'FORM'.
Lastly google have stated that sometime this year they are going to adjust ranking, they will virtually ignore non HTTPS sites!! I think this is some kind of deal they have done with governments etc for favorable tax incentives. Not really sure why, google say its to make life harder for hackers and better protection for online shoppers. But again since when was it a indexers job to tell people how they should do things? To me google is like a upperty receptionist that sleeps with the boss, too much say in too many places for all the wrong reasons.
But if you think general searches are bad..........try searching videos, you would think there is only one video platform (You Tube), but there are many! Google owns you tube so the vast majority of returned results are all you tube. Whats really bad is, i have some videos I published with another platform, i can directly input the titles and google comes up with NO returned results, i look at the back end and google does regularly visit the videos. But it does not index them in searches!
Google stopped being your friend a while back, most didnt notice. Wait 12 more months and many the sites you know now wont show up at all.