For basic users - Any reason to install mobile security App? For users that play it straight, never going to doubted sites, never enabling APK from non-play sources... (e.g typical seniors) 

Any reason to install a mobile security app?

Because


Such users always surf magazines and news sites. Maybe YouTube.
As for Trojans: Official APKs are inspected by Google. Non official APKs are disabled by Settings. Seniors hardly install what they don't know. It's mostly WhatsApp, maybe some 2D game from Play, and that's all.
Worms &amp; vulnerabilities: If your phone is old and stops getting security updates, nothing can help you. Antivirus apps are apps. Hence, they can't protect you because they can't monitor and filter traffic.


Under those assumptions, for senior users, Antivirus for mobile is nearly useless. What am I missing?

Thanks
tl:dr Yes, you need such apps and more a wholistic approach 



Breaking down the possible threats (simplistically) :

1.Viruses: They don't exist on Android. See Is an antivirus really needed for Android?. 


malware : Yes a very real threat that is usually piggy backed on apps or users being lured to click spammy links and download payload. 


Since you mentioned simple games, to see the possible threat, see Flappy bird malware
You are also making an assumption that reading news sites is a safe. Not true. ANDROID WARNING: Terrifying malware can spy on almost EVERYTHING you do on your phone says




  The second one was the preferred vector: we found several news websites that have been hacked by the attackers to redirect visitors to a downloading site that serves malicious APKs


(Emphasis supplied) 

How to counter this threat is quite beyond the scope here, but suffice it to say you need protection both in terms of user education and apps. 

Personally, I find Addons Detector to be reliable for app related threat. But this scans apps after they have been installed so a better approach would be to scan before installing by using Yalp Store which lists all the Play store apps and importantly gives out a report from Exodus scanner (which you also can use directly via its website). Another way of doing pre-install check is submitting the app apk to Virus Total (Thanks to Izzy's suggestion) 

Several malware detection apps are available and it is upto your discretion to choose. 


privacy:  Apps invading your privacy is another big threat. Apart from choosing open source apps which reduce this possibility, real protection often requires rooting your device, which is not what an average user does. 


Tangentially, as one grows old, convenience takes precedence over security and apps can easily invade their privacy (permissions). 


Your second bullet point is highly debatable. You are assuming that Google Play scanning is robust and therefore offers adequate security.


This is simply not true. A Google search for play store apps removed shows at least three instances in last six months when apps were removed from Play Store. One may say that this is evidence of Google tightening security but OTOH your target group of senior citizens may have already been affected! Quoting from one report picked at random to emphasize this


  The report also adds that some of the apps dated least a year back and had been downloaded million of times.


Alternative is to install open source apps from f-droid. I do agree that equivalents are not always available but such security apps discussed earlier  have a role in supplementing defense.

Choosing open source apps and user education reduces  risk partially at least along with selection of right malware apps. 

What started as a comment, turned into a long answer (and incomplete). That said,  premises of Google Play Protect screening being robust , assuming news websites to be safe or downloading simple games as harmless are not necessarily true as proved often, time and again. 



Additional information


Trackers packed into apps are a threat and while ways have been suggested in the answer, for additional information see What’s it all about those modules apps contain? 
App permissions is another troublesome aspect and is beyond casual users, much less target users of senior citizens. For a primer on permissions and to see how how crazy, it actually is, see The Android authorization model: a perfidious construct 


Sourced from Izzy's blog (thanks)