With a full deck I indeed a deck with every basic card in the game (reward cards not included)

Do players actually do that?

Lol, I think a lot of people do.
My alpha deck I keep as a collector's item; so it had to be complete, but except for my gold foil deck, all decks I have have each card of each splinter; I even try to get them leveled up to the level the according level of the summoner.

Epics and legendaries are expensive indeed, but remember, I've been playing since season 1; and they used to give out free booster packs as a reward for completing the DQ ánd as season rewards. That means that I got the chance to build out the main part of my decks for free.
And I would be lying if I said that I didn't spend quite some money too.

After they realized they weren't selling any booster packs anymore, they switched to the reward card system. As a result, it is way more difficult for anyone who has started after - I think it was season 3 - to build their deck without investing.

It makes me feel less frustrated about the closing doors on the game. But as I mentioned before, I'm sure other people feel turned off too, and let's face it: if they stop coming the sm team will have to implement other changes. Seeing how dynamic things are, I wouldn't be surprised about things getting a bit less restrictive.

I hope with all my heart that you're right, but I'm afraid it won't. Like I said in the post, I had a direct conversation with one of the founders, and from what I understood, they are only going to evolve into an even more 'play-to-play system'.
But since so many changes have been made since the beginning, there is always hope. If enough people keep fighting the same losing fight I am, maybe it'll change for the better, maybe it won't. We'll have to wait and see.

Anyway, If you haven't read it yet, I suggest you hop over to the account of @rentmoney.
He's a big player, but with a heart for newcomers. He puts up a lot of suggestions for changes that would make the game more attractive to more people than only the top players.

His last post, Steem-Monsters: Prize Adjustment Suggestion For League Payout/Future Tournaments ! has a lot of valid arguments in it.