Because since Sam changed Al's future, would he still be friends with him/be his hologram? Or would there be no Quantum Leap, and Sam be by himself?
While a question like this is inherently speculative, they probably would have still been friends.
There's long existed a script of an alternate ending in which, after the final leap-out, Al discusses Sam with Beth.
CLOSE ON BETH
She catches her breath and tears flood her eyes as we move to....
SILVER FRAMED PHOTO OF YOUNG AL
sitting on the mantle. We hold for a beat and pull back past another photo. This one is of Al, Beth and four older children, our move takes us past other family photos of Beth and Al and their children. We continue until we reveal that we are in....
in his home at Project Quantum Leap. It is the year 2000 but this room is a classic den with leather and wood and a warm, comfortable look. Our move continues until we find....
THE OBSERVER AND BETH sitting man overstuffed chair. He’s smoking a cigar and staring at a silver framed photo in his hand. She’s sitting half on the chair and half on him. Beth’s older and her hair is streaked with gray, but she’s still a radiant beauty...especially when she smiles.
OBSERVER: Wherever he’s leaped, Sam's still himself.
BETH: Because no one’s in the Waiting Room?
OBSERVER: There’s no other explanation. (beat) Ziggy’s starting a nano-second search in the morning but I got a feeling Sam’s leaped beyond his lifetime?
BETH: Into the past or future?
OBSERVER (firmly): The future. Don’t ask me how I know, I just do. (beat) He’s in the future, way in the future...far beyond his lifetime.
BETH: How’d he get there?
OBSERVER: The bartender sent him.
BETH: The bartender?
OBSERVER: Why not? Anyone who has the power to leap Sam through time can be anyone he wants to be.. .a bartender, a train conductor...a steambath attendant.
Beth takes a second to absorb that, then looks down at Al.
BETH: He’d know where Sam was in the future.
OBSERVER: How do I ask him? As a hologram, he couldn’t hear me.
BETH: If he’s God, I think he’ll hear you.
OBSERVER: Good. But without Sam in that bar, I can’t get there.
BETH: You could if you leaped.
CLOSER ON BOTH
The Observer looks slowly up to Beth, realizing she’s hit on the solution.
OBSERVER: I might not come back.
BETH: You’ll come back. Anyone who came back from Vietnam can come back from anywhere.
OBSERVER: Thirty five years and you still amaze me.
He pulls her into his arms and passionately kisses her. Then, he’s out of the chair and gone.
Some production shots recently uncovered from a fan suggests this scene (or perhaps some other version of it) was in fact filmed, though never used:
Furthermore, typically because of Al's unique connection with Sam, he retains some level of awareness of temporal changes... even if leaping had caused him to normally never meet Sam, he likely would have retained some memories of the alternate timeline.
And from an out-of-universe perspective, if the show had continued, it likely would have continued with both cast members and maintained their friendship and rapport.