All it would take for someone to take out that engi is one well placed sniper bullet or backstab, or an uber charge up there. Yeah, it's tough to get into their base with an uber, but going through the sewers it wouldn't be impossible...and remember, you wouldn't NEED the uber up there, you'd just need it to get you there. Wrangled sentries won't auto-fire on you, so you could easily take those engis by surprise. People who are QQ'ing about it are honestly just not willing to switch classes and try to take it out.
Looks like I'm going to have to be in the minority again and say that I agree with everything you're saying. Anyone who says anything like, "Oh you just have to snipe/backstab/kill the engie and you're good, totally easy stuff" has obviously never played in a game where this was actually going on with 4-5 sentries.
I don't know about this 4-5 nonsense, 'cause I see only two sentries in that picture, and the best part is they're both wrangled. If you were a rifle sniper, you'd just peek out around that corner, likely unseen by the engi. I don't know if you've actually used the wrangler before, but it's not easy. The shield bubble and the gun itself really get in the way of you seeing what you're shooting at, and if a sniper just barely peeks out from a corner to knick you in the helmet, you'll likely never see it coming. You can use the cover there to only expose yourself slightly before picking off one engi, then his gun, then the next engi, and then his gun. You and I both know that 4-5 sentries on 2fort might happen once in a blue moon, and that's just 'cause the team with the sentries is much much much better than the other team, in which case you're complaining about the wrong thing. In that situation, just sit back and wait. The team getting rolled will have people that give up and leave, then team balance kicks in, and within a relatively short time (if you're patient), the teams will be better. It may take you a while...for example, I was in turbine last night, and BLU had the middle area DOMINATED with sentries, at least 6 of them. RED never had a chance...why? Not because there were sentries, but because they had already fallen so far behind that they were playing defensively the whole time and never had a chance to do any damage. After we won, guess what happened? Team balance had switched so many people from BLU to RED that RED turned around and did the same exact thing to BLU, though not as dramatically thanks to teams being more evenly matched. Long story short: Are you losing because there are two little wrangled sentries on the battlements and there's simply no way anyone could possibly ever destroy such a magnificent and powerful machine of death and mayhem? Or is it because your team isn't coordinated/skilled/willing to do what is necessary to win?
Every time a class update comes out, there's always SOMEONE who will say it's imbalanced and it wrecks the game and blah blah blah, but honestly, chill the heck out. It's been out for less than a week, just give it some time for people to develop strategies, for Valve to re-balance anything that the community as a whole (not just you) has decided -- in a fair and impartial way, I might add, not like this post -- to be out of balance. Since when has Valve released something and said "There, it's done, we're never changing anything or working on this again." Remember the backburner? That thing was nerfed and buffed like 8 times before it got to what it is now.
I, personally, think the engi items are pretty balanced. This is my logic: No one cried that the engi was either under- nor overpowered before. He was a great utility to his team in almost every situation, if played well. Now we have new items. Even though they are new and fun to play, the fact is that I constantly switch back and forth between items (only exception is frontier justice perhaps...I do loves me some crits) because different situations, maps, and objectives call for different loadouts. That tells me these new items are pretty equal to the old ones, if I'm constantly switching back and forth between them. That's the idea of the updates: Add utility, expand the role of the class without stepping on the role of other classes, and obviously give the player customizability and a unique, fun experience.