The USS Lakota (an Excelsior class) was upgraded as it was noted in 'Paradise Lost' (its shields, weapons and engines underwent upgrades - internally) ... so, it stands to reason, all other Excelsior class ships underwent same upgrades prior to the war
This is a flawed argument, as the Lakota was the only one that had this newer configuration. It clearly didn't cut the mustard, as literally every other member of the Excelsior class seen since that time was of the original configuration. If it was all that and a bag of chips, every new Excelsior off the line would have the E-B/Lakota configuration. This was not the case.
otherwise, I doubt SF would be stupid enough to send technologically outdated and underpowered ships into combat against an enemy that would have otherwise thrown rather 'modern' ships at them.
Remember the multitude of creaky kitbashed ships that they threw at a single standard Borg Cube at Wolf 359 that got literally every last one's ass handed to them? Or the DS9 Frankenfleet made of a hodgepodge of ship parts thrown together to serve more as white noise against an oncoming aggressor than a viable battlegroup? Yeah, I 'member...

SF did that all the freakin' time if they didn't have the resources - It was almost state-sanctioned mass suicide!
The resulting upgrades put that Excelsior effectively on par with the Defiant... had it NOT been upgraded, there would have been NO CHANCE at all that the battle between the Lakota and Defiant would have ended like it had (in a tie)... it would have most decidedly been in Defiants favour otherwise... and the battle would have been over far faster if the Lakota wasn't upgraded.
Again, if the newer Lakota design was so awesome, why was no other Excelsior shown with this configuration? Because of this, there is more evidence to demonstrate that it was a failed experiment than it was the first of future plans. It was a sad victim of mission scope creep, retrofitted to engage in mission profiles that it was never originally designed to do.
The Defiant had the extra benefit of ablative hull armour... but it was also overpowered for its size. The Excelsior however had a much larger frame (FAR larger), and even if it lacked ablative hull armor, it likely made up for it in shields and better power distribution systems that were able to support a large/more powerful/modern warp core which was able to maintain shields for longer periods than what the Defiant could (comparatively speaking).
Agree.
We also saw other Defiant class ships in some of the fleet scenes during DS9... so the Defiant wasn't the only one.
Yes,
later on, proving my other point that, since the Defiant was a more successful design, it was worthy of duplication. The Lakota ultimately was not. I also suspect that such a comparatively vessel took too much resources to build, when they could easily build more Defiant types for an equal "bang for the buck", as it were, probably at around a 3:1 or 4:1 ratio (much like 20th century pocket battleships). Abandoning Lakotas for Defiants during a time of war made good economic and resource sense.
Plus, SF pretty much abandoned working on the Defiant when the Borg threat became less urgent... so it was mothballed. Sisko was the one who pulled it out and over time, the ship was 'stabilized'... also, they never really found a solution to the overpowered factor of the ship... all they did was basically keep the ship under Warp 9 at most of the times to prevent issues.
Sadly, this proved how short-sighted SF was to the Borg threat - which never really ceased being a threat (until, ostensibly, PIC S3). Q bent over backwards to warn them what was out there, and they really didn't listen. The Dominion was just another in a long line of bullies to rattle SF's cage. The Federation's high-minded goal of exploration is all well and good when the wolves aren't at the gate ("It's easy to be a Saint in Paradise" -Sisko), but they seemed to be in a quasi-denial state as to what threats existed out there. It's always been their biggest blind spot. They always assumed (naïvely, IMO) that, if a species is intelligent enough to master interstellar travel, they must
obviously embrace diplomatic principles first before resulting to war. Should diplomatic principles always be the first step to establishing a relationship with other such powers? Most certainly! I would never advocate otherwise. But when it is proven that said power is openly hostile, I might recommend a quicker pivot to a defensive posture, lest the Federation be perceived as weak (which has happened
countless times in Trek history).
At any rate, for SF to be able to bring up an ageing class of ships all the way up to the Defiant's level in tactical ability was nothing short of superb with nothing more than internal upgrades mostly... I actually wonder how powerful the Excelsior II is in the early 25th century.
Agreed - it was an interesting strategy, probably to serve as a stop-gap measure to massively upgrade existing platforms to stem the tide of a threat until such time that enough mission-specific platforms like Defiant could be moved to the board to decisively turn the tide and finish the job.