Despite how emotional a lot of Arsenal fans are feeling, I find it hard to point the finger at Emery too much at the moment. He's had his hands tied all season, both on and off the pitch, so it isn't fair to be so critical of him when he hasn't been given the environment/resources to do what he needs to. He isn't faultless, but at the same time, there's only so much he can do.
5th and a Europa League final isn't a terrible first season - in fact, I expected far worse at the beginning due to the sorry state the squad was in - but it is heartbreaking to be so close on two fronts to getting Champions League football, yet falter at the final hurdles on each occasion. The expectations of the fans had risen throughout the season and we've come back down to reality with an almighty thud. As you said, financially, the implications are huge.
That being said, this season has been an unmitigated disaster for various reasons.
Our manager was not backed properly in any of the transfer windows, Sporting Director quits as his vision doesn't align with the two newbies at senior level, contract situations (Ramsey and Ozil) have been a total mess, 2nd major shareholder grows disillusioned and decided to sell up to Stan Kroenke, fans forced to give up their shares, we're still no closer to appointing a new Sporting Director 6 months down the line when we knew he was leaving from December 2018, fall 1 point shy of obtaining Champions League football via the league, have another shot in the Europa League final, in a horrendous location, and get absolutely destroyed by soulless London rivals...to compound our misery, the owner doesn't even bother attending our biggest game for 15 years, which reinforces just how low down we are on his priority list.
We are only going to drift further away from the pack.
An accumulation of bad decisions, poor management, lack of strategy, lack of ambition and lack of leadership will eventually be damaging to any club, especially if other clubs are doing things right.
It's a long, hard road ahead. We will be competing with West Ham for 8th/9th spot in the coming years. Just you watch.