I am not yet worried about Wolverine being in the film, but I don't get this notion that anyone should be automatically "happy" about decisions that mean big money for the movie, unless you're talking to Fox executives. What matters in the end is whether the movie is good and if some people are worried that it will be a bloated mess I don't see why they should care about the box office numbers. That's Fox's job.
I'm (obviously) extremely bummed at the news of HJ's potential return (and irritated at all the commentary that Wolverine is a surefire way to make money/is the main draw for this film, or that his brief cameo in XMFC making people laugh is proof that people are desperate to see him headline a SEVENTH bloody movie). The only glimmer of hope I have about this is that he is "in talks". If he were to have a major role, such as the traveler, I doubt that he would be "in talks", but rather would have been cemented down before any announcements were made. That way if he had to back out they wouldn't have to scramble around trying to decide whether they were going to go back to the original Russel-Crowe-as-Wolverine plan or if they were going to have to suddenly rewrite the entire script.
I'm torn because every announcement about this film has been a huge disappointment and at this point in time I don't actually want to support it and thus feed the idea that only an X-Men movie with the old cast is a good one. But at the same time I care about it making money is because I want it to do well enough that they decided to go on and make FC3, which would hopefully be an actual FC sequel (triquel?) and not X-Men-does-Avengers-with-time-travel.
The unfortunate flip side is that they could be planning to expand this storyline into 2 movies, which I really hope is not the case. You make a valid point, however, on the possibility of X4 dwindling as the years pass. The actors are not getting young and as you said, the "sendoff" is much less open-ended and hopeful than X3's was.
On that note, there's already a huge issue with the timelines even for DOFP:
Option 1: If their apocalyptic future is one that happens post-X3, they need to explain why it is that Xavier is alive and in the same body and walking around. What happens post-credits should not be an important canon element (such as BRINGING A CHARACTER BACK TO LIFE), as most people leave the theatre before the credits roll. And in some cases, what happens post-credits is downright contradicted (see: Thor). In addition, I'm not sure we ever get told that the Patient X (or whatever his name was) is Charles' twin brother. But if we are, I think we probably should have seen or heard about him in First Class. Shoehorning in "er uhhh braindead twin brother. Yup." is not a wise plan; this is something they needed to have touched upon even in an offhand comment in First Class. Not to mention, his moral ambiguities aside, I have a hard time believing Charles would take over another person's body, braindead or not, for the sake of his own survival.
Option 2: The "happy ending" future is X1-3. Ughhh what a nightmare this one would be. If part of the reason they're doing DOFP is to fix some of the more massive continuity errors, they just can't go with this. Charles walking? Mystique not knowing Charles (because it will be forever impossible for me to believe she could ever remorselessly hurt him)? Beast human? Charles being mystified by Erik's "telepathy shield", or having met Erik at 17 instead of late 20s/early 30s? It's just not going to fly. People, ordinary non-fan audience members, did notice and WERE annoyed or perplexed by the inconsistencies. Let's not alienate them further.
(the rather unlikely) Option 3: The apocalytic future is a whooole other future and this IS kind of cementing the notion as XMFC as a pREBOOT. The "happy ending" future remains unseen (can be presumed to NOT be X1-3), leaving the door open for even MORE FC films beyond the first three. Although with their rapidly increasing in-demand status, I'm not sure Fassbender or Lawrence would be up for more. Or James McAvoy for that matter, who despite seeming to love his role as Charles does tend to opt for movies that are more film festival gems than action blockbusters. And an X4 would require more mindbending as audiences try to figure out which timeline/potential outcome X4 is following. No X-Men movie should have to come with a pre-show explanation or a timeline map. This is X-Men, not Donnie Darko.