So, will the Tier 5 raids still exist after 2018?

I think it's marginally possible they we may go small periods of time between tier 5 raids. E.g. that when one tier 5 ends, a new one will be announced but may not start until a week or two later.

Even that, I think, is unlikely.

Gen 4 has a load of legendaries, and with each one typically getting a month run on release, in addition to desired reruns of groudon and RayQ, it will be a really long time before we get through them all. I.e. beyond the start date for Gen 5. It already looks like Niantic will need to either shorten release Windows or run more than one at a time - so not running any seems real unlikely.

So do you really think they will remove the part that generates them the largest income. Sounds legit

I don't expect that Niantic will stop Legendary Raids until they can replace that source of revenue with something else that is just as lucrative.

Money. Income. Profits.

That answers your question.

https://www.pokemon.com/us/pokemon-news/2018-will-be-legendary/

The Pokemon Company has been running legendary promotions all year. Niantic was just tying their own announcements onto that for continuity. With or without the Pokemon Company's legendary events, Pokemon Go will still have T5 legendary raids.

As others stated above, T5 raids draw too much money for Niantic to completely abandon them.

There's also a lot of legendaries to go through that haven't been released yet; 8 in Gen 4 (9 if you include Giratina's Origin forme), 9 in Gen 5 with some having multiple forms, 3 in Gen 6, and Gen 7 gets messy factoring in the Tapus, ultra beasts, the fact that Solgaleo and Lunala are evolved, Necrozma having 4 different versions, and that TPC considers Type: Null and Silvally to be legendary. These are all before considering re-runs of previously released legendaries which will pad out the raid calendar.

Hopefully some of these get sped along either by shorter availability duration or multiples being released at once. I'm not looking forward to slogging through three months of the lake trio and a month of Cresselia next summer.