>Every physical store is a monopoly in their own space.
This comparison is disingenuous. There aren't only two big physical stores (and maybe a handful small stores hardly no one knows about) in the entire world. If 99% of all physical stores were a Walmart or Costco you could compare but luckily this isn't so.
>The fact that Apple gate-keeps their store is also a major selling point of the iPhone.
That is beside the point. Just because something is a feature you (or most) like doesn't mean it is legal or not, monopoly or not. I'm not going to discuss if it is a good or bad feature because it would be off-topic.
>Monopolistic is when Google won't allow your Travel Planner to enter their search index, or deranks it in favour of their alternative.
Consider this: You have 100.000 result on Travel planners today with your site being number 3 and tomorrow you also have 100.000 results with your site being number 3 but now there's a big box above all the results that tells you what you were looking for (and the data might even come from your site). This is way worse than being de-ranked. That's not fair competition.
Another example:
You have one of many Fitness Apps for iOS. For years Apple can see data on just what people search for, install, etc. and then one day they use all this data to create their own fitness app. Quickly your app would be irrelevant. Add to that that we have proof that Apple abuse their position by offering to buy a successful app and if refused they harass the publisher in different ways and create their own.