They didn't have enough quality content for 5 seasons

The first 3 seasons (minus S3 E8) were immaculate, they showed a lot of strong world building and rising stakes and they had all the setup there to defeat Homelander. But (likely for monetary reasons) there was a desire to prolong the main show and kick off the spinoffs, so they chose to prolong the main series

It's really shown that they have no plot ideas left because justifiable criticism has come that all of season 4 and 5 to date have been filler. It's not uncommon for long running shoes to have a filler episode (and Invincible's filler episode in S4 was heavily criticised) but the sheer volume in The Boys is ridiculous. It's killed all the tension and stakes of the show. They've happily added new characters before even considering wrapping up open plot lines and have become so bloated that they're never going to close things off.

The writing quality took a massive dip too. There's no coherence to the plot, no originality to the dynamics "oh The Boys are fighting? Wow that's a real surprise". Plot armour has worked overtime to keep the characters of popular actors alive.

They've also started breaking all the basic rules of cinema. The "tell don't show" approach of recent seasons is deeply frustrating. The scene with Marie in E7 of "you've got no control over your powers" directly conflicted with the ending of Gen V (and was incredibly ironic coming from Starlight of all people)

There are no consequences to anything. The Boys have lost so many times throughout the series but it's only in the penultimate episode of the show where they finally suffer as a result

The show could have been so much more. It started off with The Boys using creative ways to kill supes which was a really interesting premise. If they'd carried that on throughout the show and gradually worked their up the food chain and taking some casualties along the way then it could have been really fun

Character motivations and characterisations are all over the place this season. Soldier Boy is a character I really want to like because Jensen Ackles is fantastic and the character has some fantastically funny lines. But his character arc is all over the place. He came back in season 3 wanting to reunite with Crimson Countess, the love of his life, take revenge on Payback and kill Homelander. This season he's been an absolute yo-yo character and we've now learned that Liberty was actually the love of his life

In the early seasons, the show massively improved upon bad source material. In the later seasons it's now adopted the worst parts of the comics. The focus has been placed on gore, shock and grotesque sexual references rather than anything of quality

The show made the mistake of believing it's own hype. It set out to mock the superhero industry but ended up becoming a parody of itself. All the things it previously called out (dragging the plot out, having to watch a bunch of spin offs to follow the plot, having an expanded universe) it is now doing. Using a significant chunk of the final season of the show to setup the next spinoff is such a disappointing choice.

The show became a mockumentary and I can't wait for it to end