I'll try and answer the bolded key points in order.
Scoring will be better despite losing Bertuzzi and Vrana who barely played for Detroit last season. Debrincat for Kubalik is a clear upgrade and we're likely to see DeBrincat used as the primary weapon next to Larkin this season. In addition to this they added Daniel Sprong who scored 20 goals for the Kraken, Klim Kostin who scored 11 goals in 57 games for the Oilers, and Christian Fischer who scored 13 goals for the Coyotes. JT Compher scored 17 goals for the Avs is a clear upgrade in every single facet of the game to Pius Suter, the previous 3C. Hronek for Gostisbehere is pretty close to even. Then there are at least 2 forward prospects and 1 other defense prospect that could be called up in the season to provide some offense in Carter Mazur, Marco Kasper and William Wallinder. Yes, Detroit will score more next year.
As for Husso and why he fell to earth, he was forced to play with an injury when Detroit's backup options were Magnus Hellberg and Alex Nedeljkovic. Lyon is a clear upgrade on Hellberg. I remain skeptical of Reimer as an upgrade over Ned, but I really doubt that he could be worse.
On paper they looked fair to middling last year and were competing for a playoff spot into February before the wheels fell off. Part of this was the untenable situation with Vrana's suspected substance abuse issues keeping a top sniper out of the lineup. Nobody was anticipating that Bertuzzi would end up with TWO broken hands. Or that Ned would play himself into a minor league demotion when he was a Calder finalist 2 seasons before. Or that Rasmussen would break his kneecap while having a career year. Or that Chiarot would be the absolute worst partner for sophomore Moritz Seider (although fans on this board called that out after game 5 of the season...). The offseason acquisitions this year are much more in line with Coach Lalonde's and GMSY's long term vision of a hard to play against defensive hockey team.
Onto defense, Detroit's third pair of Robert Hagg and Gustav Lindstrom may have been the worst third pairing in the league, and the worst Detroit has iced since we saw Dennis Cholowski and Madison Bowey there. Adding Holl to the third pair and adding Edvinsson to the roster for the year are very clear improvements to the team's defense. And even Gostisbehere is a (slight) upgrade to Filip Hronek. Furthermore, Detroit replaced their 3C, Pius Suter, with JT Compher, which I think needs to be mentioned again. Compher was a pretty important defensive player for the avalanche last season and received Selke votes. I don't expect 50+ points from him, but 40 and playing key shutdown minutes, PK?
Last, Yzerman made his statement after a very disappointing end to the month of February at the trade deadline, not July 1st. It was meant to be a signal to the team that there would be a lot of changes, and there were. Detroit upgraded every position this offseason. I'm expecting a 20+ improvement in GF and a decrease in GA by at least 20. I don't know if that's good enough to get them into the postseason, but I certainly think it puts them closer and will keep them in the discussion for a playoff spot longer.